It depends on what you call a "cheap" stratocaster. If you are talking about a squire that came in a "strat pack", it may be worth upgrading. I've read where a lot of people are more than happy with their MiM (Made in Mexico) strats. Others swear by their Squire strats...it really comes down to a matter of preference.

Issues you may have with a cheaper instrument are poor action, inability to stay in tune for an extended period, switches and knobs get noisy and flakey.

If you like your strat, I'd say the best thing to do would be to go to a local music store that would stock higher end American made strats. Try some out and see what you think. You'll either be "OMG, why didn't I upgrade sooner, this is awesome", or you'll be "meh, it seems pretty much like my current guitar".