Good luck with that. Alien technology would have to sell for almost nothing in order to preclude this sort of situation, because not only do I max out my cash, I can roughly do so several times over. Downgrading the prices of the alien gear in response to the frequency of selling might work, but realistically, would the market for such novel weaponry consign them to the bargain bin after the sale of hundreds, even thousands? I think not (the world market is huge, that's less than a drop in the proverbial bucket). Even assuming that the rest of the world learned how to mass produce such technology, it would always command some sort of premium over conventional gear at least, and given the amount of weapons you recover per mission, coupled with inflation, which would also work in your favour, this is not a particularily viable solution to the issue. Inflation regardless of the amount is also for this reason rendered insignifigant. These problems readily exist even discounting profits from manufacturing.
The way I see it, you'd have to completely overhaul almost everything about UFO:AI's economics. Nerf national contributions, alien weapon yields, and increase non-equipment expenses (because increasing expenses for conventional gear means you have to increase yields for superior alien gear accordingly). Either way, inflation isn't going to have much of an impact, because you deal in alien tech, which always benefits from it, which you receive in copious amounts absolutely free, and which is always more lucrative than its conventional counterparts.
Basically, no matter how you slice it, inflation fades into complete irrelevance in terms of gameplay.
EDIT: Added minor clarifications.
