I'd suggest getting a part time job (from what it sounds like, you and your buddies haven't graduated school yet). Between the three of you, you should be able to accumulate the $140USD fairly quickly. If you are (and your friends) are already working, just set aside (each) $20 USD a week. In about 2 weeks you will have enough to purchase it.
Unfortunately if you can't find $140 you can't buy XenForo. As harsh as that seems, that's life. Products have a price and it just so happens this product is priced competitively and discounts aren't appropriate as a result.