Lots of retro-fitted Bow thrusters. Normally Vetus whereas first fit are normally Sidepower. Personnally I prefer the Sidepower ones, Cost will depend on Model, and lots of boatyards will be very happy to quote and fit.
If you have single screw boat, then a bow thruster is a very useful tool in a crowded marina. If you have two engines, then it becomes less of a requirement.
I had one fitted to a S34 by JS mouldings in Hamble. Very professional job. I think the cost was about £2750. 4h.p version. It paid for itself in a year on preventing potential damage on windy days. I went cruising on my own in France and it was a doddle berthing.