I can't speak about the other vendors but Budget had pickups for 800 baht / day when I rented my car. If you rent longer than 6 days there is no charge for returning the car at a different place from where you picked it up. I had my car for 4 days and paid 1000 baht to take it from Chiang Mai and return it in Bangkok.
Their prices include basic insurance and you can purchase additional insurance on top of this.
Check for special pricing here:
http://www.budget.co.th/hotdeals_home.php
Good luck.