Yeah I remember the build-for-profit in old X-com
For the sake of realism it should have had a major setback (if ever allowed, not in this case): If you sell powerful weapons not aviable for the rest of the world you may face humans using those same weapons if they turn against you.
In XCOM apocalypse, ths was balanced by cheapening weapons every week you sell, and increasing prices in the ones you buy. However, after raiding the alien homeworld the game goes anticlimax.
Since there are 9-10 alien structure missions, and completion of each would make the aliens lose severe advantages and force them on pure defensive (especially after icing the alien queen).
Also, The FIRST Alien lair would yield enough artifacts for gazillions of dollars, eliminating ANY need for money.
If you could completely purify Mega-Primus, it became a matter of shooting down produced ufos every monday, and grinding towards the final building.