I make a lot of home brew magic items (Some times on the hoof while DM'ing which is why one of my players now has Elton Johns sparkly rose tinted glasses which shows you what things really are, sans magic) and working out what something should cost is a nightmare; So I've been tinkering with an excel sheet to try and make a simple way to get an estimate for the value of a magic item:
There are 6 standard and 3 extra drop down menu's (on the left) which should give you a value (I based the values on a sane magical prices index, as its a sheet I can just look through easily) Its a work in progress and it needs testing, if people would like to play with it and give some feedback it would really help :)
Home Brew Magic Item Cost Estimator