For day boats then the beautiful craft available at the Beetle & Wedge http://www.beetleandwedge.co.uk/boats.htm are a joy to behold. They run out at £45 per hour weekends and £35 per hour week days. Special rates for half/full day.
Depends how far in a week you want to cruise and which part of the river you want to explore. Plus your preferred start/finish point. Kris Cruisers at Datchet have a very smart fleet. I would imagine prices are pretty standard up and down the river.
How remiss of me. YES! Kris Cruisers do have a very smart fleet indeed and as you rightly say prices will be pretty standard although you might be able to negotiate something with a smaller firm.