Reading the plan terms on prepaid service I have found :
Boost limit $300
Verizon limit $1000
Cingular GoPhone Pick Your Plan limit $500

Does anyone know if t-mobile togo, cingular KIC tdma or gsm have a limit ?

This becomes a issue due to adding money to rollover the balance before the
expiration. If you are getting close you may increase your use to avoid the
breakage/lost of money.

See More: Prepaid balance limit