I'm still somewhat new with VCE, but I believe it would be:
0 OnActivate | Self | VCE_ifVariable | <var:(target money is bound to):(name of money variable)> | >= | (Amount of money it costs) | 1 3
1 OnVariableTrue | (target money is bound to) | VCE_Modvariable | <var:(target money is bound to):(name of "money" variable)> | Subtract | (Amount of money it costs)
2 OnVariableTrue | Player |
SetInventory | (weapon) | (slot)
3 OnVariableFalse | Client | Chatmessage | "You don't have enough money to buy this."