Hard to say if you're paying too much without actually seeing it AND the neighborhood. Remember the first cardinal rule in real estate - location, location, location. Next biggest factor? Condition of the house and any extras that make it different from surrounding properties. The ultimate factor, however, in determining price is the real estate market itself. People and things don't set the asking/offer price, only the "market" itself sets the price. Lots of factors go into the market equation - ex. local unemployment rate, price stability of consumer goods, economic outlook for region, number and quality of other homes on the market at the same time, etc.
If you have an accepted offer and/or a signed P&S, there's really nothing you can do to change your price other than back out of the offer and possibly lose your deposit, unless you have an escape clause for either the home inspection or financing contingency. However, you're definitely in the ballpark with your offer of $288k and you just need to decide whether or not you love the home enough to complete the deal.
Best of luck! -Tim