I paid about £40 (it was $US80) for a two-year certficate a couple of months ago (from COMODO), and I've not had a problem yet with any browsers not recognising the root. There are some cheap ones around if you search, otherwise you are limited to education of the users or pre-installing trusted root on their browsers (neither of which I would relish - you could have bought a cert by the time you have dealt with the Nth e-mail saying "it doesn't work!").
The way IE7 deals with it is appalling. At least IE6 would tell you why the cert may not be all it seems. With IE7 you cannot find out any information about why it is being rejected without *first* accepting it, by clicking on the big red "here be dragons and death" link first. No wonder teachers and students have a problem with it.
-- JJ