Hi. The first thing is the cost of whichever hub you decide to go with. I guess the most common is the Shimano Alfine range of hubs but I think Sram do some as well, have a look on the net for reviews before deciding which is best for you. Second is the cost of the shifter as most gear hubs need a specific shifter and are not generally compatible with dérailleur shifters. If you buy a hub that has the same number of spoke holes as the wheel currently fitted to bike then you can re-use the rum, otherwise you will need a new rim costing between £25 and £40. You will probably need new spokes as the spoke length will be different to your current wheel. Finally the cost of wheel build. Unfortunately I can not help with pricing spokes or labour for the bike shop to build the wheel. As you are mechanically minded you can try building it yourself, there are good books on the subject and lots of info on the net. Getting the spokes the correct length is slightly trickier, I cut my spokes down from longer spokes and then re-threaded them. I bought a spoke threading tool for that job but they are not cheap and the threading process is laborious. If you think you will get into biking and wheel building then get the gear and try building a wheel, otherwise I think getting a bike shop to do it for you will be the better option.