sorry the Baht 300 estimate is for the existing airport (my mind is still set on Don Muang because I've already used it three times this month)...it will be a bit more (B500? all inclusive) from the new airport...
The owner of a Bangkok guest house made the trip by taxi from Suvarnabhumi to Sukhumvit soi 11 yesterday and posted pictures and details. It cost a grand total of 306 Baht including dispatcher fee and highway tolls.
Here's his posting.
It's a myth that the new airport is all that much further from downtown than Don Muang. It's about 5 km more at the most. It's also incorrect that you'd have to pay a huge amount extra if you get caught in a traffic jam. I'm not sure of the exact amount for standstill, but a couple months ago I was in a terrible snarl in northern Bangkok trying to get to the Impact arena, and in about 2 hours the metre only went up about 20-30 Baht.
The jam-up near the new airport is on the Bangna-Trat expressway due to road work. The other road seems to be OK.