I should rephrase and say: Every DIYer worth their salt should pay good money for:
1. a good pair of FABRIC scissors and 2. a strong, heavy duty staple gun
If you deal with fabric, you should have fabric scissors. They will run you $13-15, but they will cut sharp and they will cut straight and they will let you cut through layers and layers of fabric with ease. Even if you aren't sewing the fabric, fabric scissors are a must if you are regularly dealing with fabric.
...and the staple gun? Regular staplers can only staple 4 or 5 sheets of paper together. For anything else, get a solid staple gun and you will have so much fun attaching fabric to wood and wood to wood and fabric to cardboard and cardboard to cardboard...and you get the idea.
If you really want to go DIY crazy, a good sewing machine is your 3rd purchase...and I've heard a cricket should be the 4th, but I'm not there yet.
...and don't flip out...a hot glue gun IS an essential, but I don't consider it to be something you have to spend a lot of money on. I've used lots of glue guns and my cheapy one has lasted a very long time and has had lots of abuse and is still trucking on.
What are the essentials in your toolbox? What can't you live without?