Depends on what you want to take images of really.  Many people use the term "macro" when what they really intend/need is a "close-up" lens. Some zoom lenses have a macro capability, but they aren't TRUE macro

A true macro (1:1 ratio) brings you really close to flowers and most insects, and the results may not be what you actually want. A 1:3  or 1:4 ratio is pretty good for flowers, unless you just want to take a close up of pistels or stamens.....even then you will only get a small bit in focus at 1:1. Similarly butterflies are really too big, unless you just want a small part of them.

You definitely need to consider focus distance to the subject  you'll be capturing. Most insects won't sit still for 50mm. I find that  my 100mm 1:1  is still pretty short and end up spooking them alot.

Tamron has rebates on many lenses til April 30th and a great 6 year warranty!
http://www.tamron.com/lenses/rebates.asp

There's also the reverse macro method...that might be an option for you as well
  http://stephenelliot.com/2007/05/15/reverse-lens-macro-photography-tutorial/