Financial aspects
Probably the question which arises most when looking into schemes is 'how much will it cost'. As a rough guide you probably will be looking at £1000 to £5000 per installed kW. The variation being down to the type of scheme, any existing infrastructure you may have, and how much you want to do yourself.
Small schemes are nearly always a little more expensive per kilowatt than bigger ones. High head (say 50m) sites can be between £1000 per kW for a largely self designed and built system (but starting with no usable infrastructure), to around £3500 per kW for a professionally designed and installed system. This rule of thumb may well be true of over 10kW schemes, but smaller systems would tend to cost more per kW - because there are some fairly fixed costs to account for.
Low head sites (down to 1m) are more numerous, and tend to be more expensive, a range of perhaps £3000 to £5000 per kW for a professionally installed system. Machinery is usually more expensive for a low head scheme with a given output than a high head scheme of a similar output - because low head schemes will need to deal with larger volumes of water to generate a given output.
The income it can generate is also subject to some variations. You will be unlikely to have the system generating at full power all the time, perhaps on average it will run at 50% capacity. The use the electricity is put to will also be important. If you are using it to lower your own bills, then the price you usually pay will be the value of each unit or kWh - this might be 10p per unit. If you are selling to the national grid, then each unit might be worth 6 - 7p.
Payback is often quite long, because of the nature of high capital costs, and low running costs. However, if you are prepared to make a long term investment, a well designed plant will continue to run cheaply and effectively for many years. Again the BHA small hydro guide has lots more information on this.