Twenty or thirty dollars might be a good deal if there is enough hair..you need like a half pound (8 oz I think?) to do a whole head with extensions. So if Sallys is selling the hair for $20 an OUNCE, that's $160 for all the hair you will need. If your hair is very thin you might be able to get away with 1/4 pound especially if the extension hair is a shorter length (altho it doesn't sound like it would be since your hair is already medium length).
When I got extensions I always got the hair off of www.hisandherhair.com
The 1/2 "mink" hair or whatever it is isn't much better than the regular quality hair...it just gives you about a week and a half before the hair starts knotting up ridiculously instead of the hair knotting up ridiculously as soon as you put it on your head.
If you get extensions, plan on spending at least $100 to have them put in because clip-in extensions NEVER look good. You can either have them braided in in individual braids (I paid $250 to have mine put in and that's the cheapest I could find). Or else, if the Sally's hair is on wefts, you have the wefts sewn onto tracks (cornrows) which are braided on your head. The stylist would have to do the cornrows and then sew the hair onto them...unless you have a friend who will do it for you for free and knows how to do it so as not to pull all your hair out and give you bald spots.
You can have them glued in, which is more expensive than either above method, and no matter what anyone says, putting glue in your hair is NEVER a good idea and your hair WILL break. You can do the thing with the mini-rings, but that costs like $500+ from what I've seen online...
Once the extensions are in, plan on wearing them in a braided style 90% of the time. If you leave extension hair loose, it tangles within about a half hour and it's a pain in the a** to untangle it. That's how poor quality extension hair is....also, plan on spending like a half hour in the shower washing, conditioning, and detangling the extensions (the extensions swell up when wet and get tangled when you wash it, no matter how careful you are) whenever you shower with your hair down. Be prepared for your head to hurt for the first few days because of the braids used to put the extensions in. You always need to use leave-in conditioner and it's a good idea to comb your hair like 3 times a day even if you wear it in a braided style....just to keep it easy to comb.
I'm not trying to scare you away from it, but I have had extensions before (Better quality hair, with an experienced stylist to put them in for me) and it wasn't what I expected. I'm not exaggerating when I tell you those things, that's really what happens. I gave up extensions when my hair was about 6" long because it was just too much of a hassle for me to deal with. If I were you, since your hair is already pretty long anyway, I wouldn't even bother with it. Good luck to you if you decide to do it, though!