Assuming those are direct flights, I think £900 each is very reasonable for August (peak season). We paid a similar price a few years ago (also in August), but felt we got a bargain because most websites quoted much higher prices so we were delighted to get the cost down to £900 each.
--- Drove Route 66 in August 2012 ---
There's no way of avoiding the one way fee i'm afraid, though I found 2 companies who charge a flat $500 one way fee (others are higher) I got mine through Alamo (via Rentalcars.com) The earlier you book the car, the cheaper it will be really. It's because they know some people will pay whatever for them if it's short notice. Same as flights are more expensive nearer the time, as businesses and companies will pay top dollar to get someone there last minute.
Good luck on the trip. We've set aside 3 weeks for our journey, but we're diverting at Kingman and up through San Fran for a while.
We are travelling out 6th September from uk. In to San Francisco and out of Houston 3 weeks later. R66 Barstow to Amarilo plus friends each end. Rentalcars.com have always been the best. At the time I booked the car, about 3 weeks ago, National had cheapest one way drop off @ $399, but it can get up to $1000 so take care. National were £20 more for car rental but overall cost saves between $120 and almost $600 so get your ideas about the car you want, get a quote or two then, either give them a call or wait until they call you. They are not hard sell merchants, they just want to help. I have no connection with the company at all, but they have sorted my car rental in the USA since the mid 90s when they were carhire3000.
We live in the midlands and had planned to use Virgin Atlantic, but after the "free" upgrade to premium economy was going to almost double the cost we are using KLM Birmingham to Amsterdam then SFO retuning via Amsterdam from Houston. £623 each.