Answers

Vinyl window replacements can range from $400-$600 depending on the brand.  Capping the windows can be an additional charge.

If you go with a Lowen, Norwood, or Lincoln wood window you could be spending north of $1,500-$2,000/window.

Anderson & Pella wood windows with aluminum cladding can be in the $700-$1000 per window range. 

Lisa, 

A project like this can range in cost.  A Good starting point for front porches can be around $115/square foot.