Are you ok with free2play downloaded games? I'm assuming you want a game in the disk form because you don't have a credit card to purchase things online with. There are many free games online that are really good.
If you want a great rts game thats cheap now, get age of empires. Its a 7 year old game so your laptop can probably run it, but I'm not sure how good your specs are.
Most newer games for pc wont be optimized well enough for old computers, so it would be difficult. Even my 1 year old laptop I bought for $400(was better than the ones $600 or less) struggles to run newer games like Guild Wars 2, Tera, Blacklight Retribution, and Smite.
run dxdiag on your computer so we can see your specs and see what it can run.