(Everything correct as of October 2017)
Bus from Armenia to Salento - 4,800 pesos
You can catch this from the terminal or as it passes through the city - we caught it by the bridge on carrera 14 (there's a mini shopping centre by it, and a bar/food places under the bridge itself). Takes about 20-ish minutes.
Willy Jeep from Salento central plaza to the valle - a little over 3,000 pesos
There are loads waiting, along with (usually) other travellers, so expect to be crammed into a Jeep like sardines. Takes 30-ish minutes.
There are two main trails - we took the longer one, which lasts about 5 hours and ends in the valley itself (the other is shorter and starts in the valley). There's a guy charging 2,000 pesos a few minutes walk into the long trail (some of the land is privately-owned) and later on there's a hummingbird cafe you can go in for 4,000 or 5,000 pesos. However, if you go to this, don't continue up the mountain like we did, because it turns out you're not allowed all the way up it. The proper trail takes a right, shortly after crossing a very rough bridge made of a few bamboo poles and a single metal wire hand-rope.
Return costs are, of course, the same: 3,000 and 4,800. The whole trip costs around 22,600 pesos, depending on if you go to the hummingbird cafe.
Sidenote: make sure to take snacks and water, plus something warm - it gets chilly up in the mountains and high in the valley. Also, you may get the chance to ride on the back of a Jeep, which is an experience worth taking but definitely not repeating BECAUSE IT'S TERRIFYING.
Next time, on AWOUND THE WORLD WITH WAYNE: a desert!