The fundamental issue is that you add Fund dice to your Resource pool, creating an exponential increase in buying power. Top that off with with a little Artha and some Help and it throws your math assumptions out window, in practice. This may not be nearly as obvious with only an occasional rare, small ticket item sale. Particularly with a very small Resource character. But it is a substantial effect.
Sell superior quality arms, or run of the mill plate and you can end up with 15D Fund. A B5 Resources character can then turn around and be odds on favorite (before Artha) to buy castle with that.
For that B5 Resource character to have a better than even chance, before factoring Artha (and Help), to buy back that armor or weapon they only need a 5D Fund. Of course it is much harder for a B2 character, and easier for a B8 character, but that is fine because the B2 is poor. Trying to hang outside your present economic station is perilous and difficult.
Likewise, a swing larger than +1D/-1D off a given amount is quite sizable. Because of the exponential nature of ability rolls and the item prices, a fixed +1D/-1D is already effectively a percentage multiplier on of the underlying linear price.
P.S. Note that the rules state a single Fund ranges from 1D - 10D in size (page 374 of BWG). Your suggestion very quickly runs over the top of that. I think it is clear why, now?