There's an easy way and a hard way. The hard way is if you're using cash.
Easy Way
onAcivate>Client>ifScore> >= 50
I dunno teh rest. :P
Hard Way [Cash] [VCE]
You need VCE for this.
onActivate>player>VCE_ifVariable [Money] [=] [MoneyAmountHere]
onVariableTrue>player>addItems [ItemName] [Quantity]
onVariableTrue>player>VCE_modVariable [Money] [Subtract] [MoneyAmountHere]
onVariableFalse>client>chatMessage ["Not enugh Cash."]