Yes it's quite a serious fault and very common too. To do it correctly the power head needs to come off and midsection stripped down. Around £160 for the mounts alone, £250 for the yoke if that's worn too (usually is). Then you have labour. It will end up around £800, unless the engineer gets bogged down with sheered bolts etc in which case it rises further.
They get like that if they've had harsh acceleration, particularly if used to ski from etc. The Merc/mariner seem particularly suspect to fail.
If its just the mounts that have worn its been known to repair them an alternate way - by cutting the old ones off (grind through the existing bolt) and re-fitting the the new ones in reverse. Does the same job but obviously should it fail completely again it would mean there was nothing stopping them falling right out, which will regular maintenance/visual checks isn't a concern.not recommended if you have any play to the yoke though.
Give me a shout if you need any further info
Peter @ Boatsandoutboards4sale ~ email@example.com
~ 07930 421007