Salamat Pagi AA,
I did learn a little Bahasa when I worked in Malaysia on the Rawang-Ipoh Double Track and Electrification Project. Unfortunately I have forgoten most of it now, about all that I can remember is 'Awak numpuk cantik malam ini' (not saying that to you of course [:D]).
I think that it won't be the cost that you will have a problem with, it will be finding someone willing to do it. The mototaxi guys probably won't know where to go, because they literally spend most of their time within 1 or two blocks of their stand.
A taxi may be willing to do it, although communicating your idea without Thai may be very difficult.
For about 15-20 minutes on a bike we were paying upto 150 Baht, but that was because we were asking them to race through traffic and the guys I was with didn't care.
Moto-taxis are generally more expensive than Taxi's, but it is rare for someone to ask them to travel more than about 100 Baht.
I think asking a Taxi is probably your best bet, although they, like buses, are more likely to be used to the elevated freeways where bikes aren't allowed.
I would suggest you have a look at Bangkok Riders
http://geocities.com/bkkriders/ I am sure you will be able to get some help over there.
I hope that you will hang around in here as well, and write up a trip report when you are finished.
Cheers,
Daewoo