Why not allow this from the beginning? Make the soldier purchase options relative to experience. If you want to invest in soldiers early on then you have that freedom but you wont as easily be able to afford R&D, base expansion etc. A top tier soldier represents many missions worth of experience so should cost a fortune, the kind of price you can't fork out early on in the game. But as you have more resources later on, a high rank soldier can be replaced. Making the soldier cost relative is better than leaving it all up to the random numbers. Also, depending how affordable soldiers are, it might encourage the player to allow a little turnover in the roster rather than keeping the original set of troopers alive throughout.