Spend your money on the best grade of paint. It really isn't that much more. Otherwise you will be paying to double coat, ie spending more than you would otherwise. Quality paint will save money and headache in the long run.

And don't try to stretch it by rolling it too far. Keep the roller loaded, roll slower, work a small area at one time, maintain your wet edge. Be nice to your wrist, buy an extra extension rod and cut it in half to roll the walls.