Depending on the model of 115hp you have, the mounts retail from 50 - 150 pounds each, + vat. You'd better hope you got a cheapie. It's not the easiest of jobs to do yourself, but getting it done by your local merc dealer sometimes exceeds the value of the motor!
There is a shortcut to doing it, but alot of dealers wont/dont know how to.
If you have the serial number I can give you a better idea of cost.
It is better to remain silent and be thought a fool, than to speak and remove all doubt!