Look at it another way.
A 7mm model is 1.75 times as long as a 4mm scale one, occupies just over 3 times the area and requires 5.34 times the volume (this is the reason why there is so much more mass and inertia in a 7mm scale model).
So, if 4mm scale is roughly 20% of the cost of 7mm, then they are about equal - ignoring other factors: if a 7mm scale model of comparable quality is less than 5.34 times the price of its 4mm scale equivalent, then it is good value for money. Since height is not usually such a big issue with model railways, then it might be fairer to consider the relationship by area, so any 7mm scale item at about 3 times the cost of its 4mm scale equivalent is comparabel, as you will only need 1/3 of the models to fill the same baseboard area.
One of the other factors is the economy of scale, and here 4mm scale wins hands down: this actually makes 7mm scale comparatively cheap, and perhaps the 5x times ratio would be fairer after all.
However, there are other considerations, one of which is the capacity of any larger scale to accomodate more detail. Some do complain that this means you
have to incorporate the visibile detail, to which I can only respond: it's your hobby, do what you like as long as it isn't complaining about the choices you made within it. So, 7mm scale offers scope for a lot more fun if detail is your bag. In which case, it is much better value for money.
Similarly, the smaller the scale the more it becomes possible to look at the impact of trains in the landscape as a modelling theme. To me the attraction of 2mm scale is that it enables this approach - see
Totnes for an excellent example which is not actually too large. Were I to work in this scale, I would very much feel the need to produce lots of trains.
If I was mostly interested in operation, then I would use 4mm RTR with a bit of weathering and detailing.
On the other hand, were I modelling 16mm scale narrow gauge in the garden, I would be happy with 2 or 3 coaches, a handful of wagons, a battery loco and a live steamer.
Total cost would be similar, but - and here's the rub - the outlay would come in larger chunks, so careful financial planning (saving up!) would be the order of the day.
And if interested in building G1 or larger live steam, I would invest in a Myford Super 7B lathe and a Wabeco milling machine: possibly slightly higher capital expenditure (if bought new - so buy second hand!) but then I would be mostly buying raw materials with the possible exception of boilers. On the other hand, if I built two of everything, I would be able to sell one to fund the raw materials cost for both!
I think one simply looks at one's disposable income, and spends it or saves it according to the modelling path that appeals the most: trains (small scale), operation (medium scale, i.e. H0 or 00), detail (larger scale) or something esoteric (any scale involving a large element of scratchbuilding!)
The more money you have to throw at the hobby, the more items you will acquire, be that RTR locos/stock, kits accumulating in the cupboard, or tools/materials. The only difference is that one potentially arrives at the chosen destination sooner, and maybe enjoys the journey less as a result (unless you simply enjoy seeing the trains running/operating): I think this is the most important thing to work out - do you enjoy the journey and are therefore a constructor or the destination, and are therefore more of an operator. You can be both, of course, in which case you probably need to get some track down quickly so that you can run trains, then spend time on the scenics, detailing, etc.
That said, there are two types of manufacturer in the hobby:
Part-time suppliers who are supplementing their own needs by making things commercially available (think how Martin Finney started out); and
Full-time manufacturers who do this for a living.
The hobby is ill-served by either of these groups making a loss, but frequently the part-time suppliers unintentionally lower the expectations with respect to price - they do not create and execute business plans (there is no need), and do not adequately cover the cost of development as they are simply producing things which they wanted for themselves, so the time would have been spent anyway. What they have done is reduce the cost of things such as phototools by spreading it over a batch of (say) 40 loco kits rather than the 2 they need for themselves.
Of course, there are some gross generalisations in there, but that's the point of gross generalisations - they give an indication of the general trend, not specifics.
As CJF said about 30 years ago, railway modelling cannot really be executed at no cost [although certain aspects can be], as there are certain essentials that one needs, dor example a power unit, a loco, some track and some stock. However, all of these are available within a standard trainset, usually affordable within a year.
What it costs beyond funding the essentials is entirely up to the individual: you have the money you have, you have the interest(s) you have. Any reasonably intelligent adult should be able to work out how to spend what they have to meet the "demands" of those interests.
The key is to decide what it is you want, and follow it - maybe allowing for the odd distraction along the way - but to focus resources of time and money on that.
To return to the important question.
How much of a constructor are you, and how much an operator?
The cost is immaterial - no point wanting to operate a large 7mm scale layout unless you are wealthy enough to afford to pay people to build it for you, and wealthy enough to be able to house it, too! But building 7mm scale models? Why yes: mostly from scratch if you have little money.