The trekking is a little different in each -- you'll see different tribes in each, and the scenery is more rolling hills in Thailand, more mountainous in Laos and more again in northern Vietnam. Thailand has by far the most developed industry -- prices are low and the number of treks heading out in any one day very high -- Chiang Mai is the major centre, but Chiang Rai, Mae Hong Son, Pai, Soppong and many other spots have trekking opportunities.
In Laos the trek often has a bit of en environmental edge to it, while in Thailand and Vietnam the emphasis is very much on the villages that you'll visit and interact with.
Thailand tends to come bundled with rafting and elephant riding -- while not unheard of in Laos or Vietnam its far less common -- particularly the rafting.
If you're looking at it solely on a cost basis, Thailand is difficult to beat. Personally I think it's a bit more interesting in Vietnam, while Laos can be a fair bit more demanding -- in part because there's fewer roads to cart the groups around on to their respective starting points.
I know the above doesn't really answer you're question, but hopefully gives you some pointers to consider. Please also take a look at the features section of the site where there's a story about how to pick a good trekking trip -- the advice holds for all three countries.
#2 somtam2000 has been a member since 21/1/2004. Location: Indonesia. Posts: 7,767
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i have been to both thailand and veitnam i would suggest vietnam for sure before things change there to much it is cheapest to organise things as you go and save a bundle ot of hanoi use kangaroo cafe for halong bay and sapa both tours together came to $200 us for 7 days including most meals it was great
#4 camstyle has been a member since 21/3/2007. Posts: 3