I had been looking for an APB at one point, and I followed them on ebay, etc for about 2 1/2 years. I saw them usually listed between $250 and $600 and watched them all. I saw the ones with seat not sell for months that were all over $350. I still don't know if they were sold, or taken down due to giving up on the sale.
The seat does make a difference, but maybe between an extra $50-$75 in good condition. I still think the price of $250-$350 is reasonable for a complete machine for a quick sale, but if you were willing to hang onto the game you may find a buyer who wants to spend more. Also consider that most people wont ship games due to the cost, so you have to factor in specific buyer demand in your area.
The APB's that will demand top $$$ will have
1) Good condition seat
2) Med Res monitor
3) All art, including chair, cp, and both sides
4) Functioning Police lights (marquee)
5) of course. . all basic functions work.