It's very expensive (around $30 or more) compared to prices in Vietnam but once you're at the bridge you forget about it. The bridge itself is amazing and the view is great! Just make sure it won't be too cloudy as you'd miss a good sight!
There are many tours organised from Chiang Rai but you can get the bike and go on your own. At some hours it gets flooded with tourists from the tours but wait up patiently until they're gone and you might end up there alone with the great view of 3 countries by the river.
Lots of fun & a great inexpensive way to spend a few hours. The mine tour is cool (it is, of course, not an actual mine). I was able to get around Goldfield with my walker pretty easily.