You can get pretty far with Tunnels & Trolls without spending a cent. Between the free abridged T&T rules and the free solo (both linked to the left) one can get a lot of gaming in. There comes a time when you want to go further, but popping down cash for the full rules, let alone on of the boxed sets, might not fit your wallet. What to do?
Head over to the Flying Buffalo site and check out the Corgi Editions of some of the classic Tunnels & Trolls solos. There are 5 Corgi books for sale at Flying Buffalo, 4 of which are double solos (the 5th is City of Terrors, a very large solo). Each book is $5.95, which is a bargain for 2 solo adventures. But wait, there's more!
Each Corgi Edition includes and abridged edition of the Tunnels & Trolls 5e rules, but they are more complete then the downloadable version you were previously using. These rules go to 11! (Think Spinal Tap). Experience Tables and the Wizard Spell List both go to level 11. With a little work, a GM could run a campaign (or at least the start of one) with one of these books.
Give them a look. They are definitely worth the price.