The palm M105 I believe is $100 now. Although you probably don't need the extra memory the m105 has over the m100. You may be able to find a m100 for under $100. I have a m105 and love it.
The new http://www.palm.com/products/handhelds/zire/
cost of $99
Or just get a Palm M105 - then you can get programs and it is a little bit more flexible.
Unfortunatley they are getting harder to find.
Zire is a nice little entry-level handheld with a very recent OS version, GREAT battery life, and rechargable too. Looks cool, has a cool case, and some nice accessories coming out...
I'd stay away from the Zire, especially with the M105 at roughly the same price point. Only 2MB of memory, no backlight and no cradle. Doesn't sound like much of a bargain, rechargeable Lithium-ion battery or no...
It looks like new 8MB Handspring Visor Neo or Deluxe go for around $90-$110 on eBay, so that might be a better option.