One of my buddies told me that I could heat my domestic water with my boiler that heats my house. Just wondering if anyone's got any information on how much it costs to put in or how efficient it would be.

It would be just about as efficient as your boiler is. Cost is high since you have to buy a special tank,valves and fittings. For a exact price you will need to get local estimates.

This depends I have been in basements and a hot water coil was in the boiler and never installed you need a local hvac guy to give you an est. but it can be done either way and it is cost effective

It is not cost effective if you are running propane or oil in most cases now days since the cost to keep the boiler hot in the summer really adds to the situation.