Hello and welcome to FF! Well, if setting up a vivarium for darts recommend an 18 in. ExoTerra cube as minimum size. That will put you down for close to $100 and the rest of equipment, plants, etc. can easily ring you another $100 or even more if going with fancy gear like controllers, foggers, water pumps, etc. An option is to watch your local Craiglist for any offers on used enclosures, lights, etc.
Your initial time will go into set-up, building false bottom, background wall, seeding substrate inhabitants, and planting. Then you mist, watch plants grow, trim any if required and let things settle down for 30 days or so.
While those 30 days are going by, you get your frogs and set them up in a quarantine type set-up (yep, another enclosure) but this one can be smaller (maybe an ExoTerra Tall Nano) with damp unprinted paper towels for substrate, a water dish, and some silk or similar plants for decor. There you will feed and observe forgs for any possible problems or diseases for 30 days. If all is good then you transfer the frogs to their vivarium.
In vivarium will need to check out all powered equipment are working right daily, clean and refill water dish, feed, and spot clean glass or plants as needed. In the Care Article section, there are 3 Dart Frog articles that go into great detail about keeping and raising them. Good luck!