If you're looking to keep it under $20 and give her options, the My Mini-Miracle has a wide range of speeds, is waterproof, and runs on AA's, which are easy to find. Don't bother with the attachments, but the toy itself is good. Aside from the "CalExotics" label, it's pretty discrete, too. Put a sticker or something over that, and it's a travel massager.
The Pocket Exotic glow double bullet is a very inexpensive old standby. I've had mine for 2 years, use it on a regular basis, and it's still going strong. This thing will not die. The downside is that while it's not loud, it's not silent either (hear it across the room, but not through a door); it's also not waterproof. However, for figuring out if she likes bullets, it's a good solid choice.
The Mini Bullet and Controller is a single micro bullet. It's only a 3 of 5 for power, but it's dead quiet and waterproof.
I second the Tantus Acute. It's a great starter dildo, and it will last forever with a little care.