America's Best Bowstrings sells strings for the Mustang of different qualities from $119 - 179. So if they were quoting on the mid grade @ 149, they're only looking at $50 for the install and any tuning.
However, I'm not sure which Mustang model that is since they made two different models with that name.
Either way, a +25 year old bow+string replacement(which may be hard to source)+the work necessary to do a basic tune isn't going to be the cheapest thing.