I replaced a couple of unbraked units on the trailer that came with my boat - Total cost was about £100 including the fixing hardware. Memory says the braked versions weren't much more expensive, unless you bought them with all the brake gear...
Most trailer places will be able to sell you a pre - drilled plate to weld on & bolt the individual units on, the fun bit however is making sure they are absolutely lined up. Not sure what your boat is, but if it's an SR4 you can probably get away with an unbraked axle, which will keep the cost down. (unless you have a particularly lightweight tow car of course!)
I think the cost of an unbraked axle probably isn't too far off a set iof individual suspension units.