Last february i took the minibus from SR to Stung Treng, think it was $14 and 5 hours. Kratie was another 3 hours i think and cost about $8 so the price you have been quoted looks about correct
I've been quoted a price of $12 for a big bus and $15 for a minivan. Was told 7 hours with minivan and a change at Stung Treng. Large bus 10-12 hours (and maybe more than one change).
That information is from today. Haven't done it myself so I can only quote what I'm told.
Thanks for the replies, folks!
Check the Sorya Bus Company