Well... if you can read a tape measure.. go meassure from the mouning areas what the comprested and extended length would be.. since they measure that on real shocks from the center eye of the mounting loop or the base of the stem.... and since one side of the shock is going to me mounted in a fixed area you can also figure out how much travel you are going to need aswell.. take those measurements and take them to your parts store.. OR take the measurements and compare them the the PDF file on Monroes site (..
http://www.monroe.com/catalog_lookup/ocl_miscappinfo.asp .. second one down) and get the part number that best matches what you need and style of shock you need.. if its more costly than what you are looking to spend then have the parts store cross ref. the part # for a cheaper shock thats in your budget.. it really not that hard...