By semi automatic, do you mean you don't want to need to rack the slide for each shot? If so, $30 will not get you anything you'll enjoy very much.
The Tokyo Marui spring pistols are pretty decent, but require you to chamber a bb after every shot. But they are very accurate, and require only bbs to operate. And can be had on Egay for about $45 ater shipping.
If you want it to be an autoloader and give decent accuracy you'll need to go higher on the cash. Anything else will either break or give very poor distance and accuracy.
For an first gas blow-back pistol I suggest either a KWA/KSC (KSC says Glock, model #, caliber... and has a plastic slide. KWA are sterile and have metal slides.) Glock 17 or 19, or a Tokyo Marui M9 or Sig P226. Great range, accuracy, and feel. But all of them are over $100, unless you find them used.