Working in Exeter I find £25 excessive - I think the taxi's would be cheaper as the journey takes about 20-25 minutes in good traffic when going through the city. Unless this is about average price for a journey of this length it has been a while since I used the taxi myself so I may be out of touch!!!
Failing that you could always get the bus to either the Holiday Inn or Park & Ride and taxi it from there. I am certain that would save a few quid as it is only a few minutes from the airport!