An SSL Cert (with some exceptions/special cases) is always bound to a domain name, so you can get away with 1 SSL Cert in some scenarios -
If the Development server has the same domain name as the live server (eg
if you use DNS to force your browser to different paths) it will work OK.
[ If you think about it, this makes sense, big players like Google who
have their content distributed on multiple boxes with the same domain
name only need 1 SSL key, even though its on multiple boxes ]
If the Development server has a different name to the live server you can
still use a key signed by a registrar, but it will throw up an error saying
the domain name being addressed does not match the one in the cert. You can
simply accept this message.
Another way to handle the problem would be for you to generate a Key for your web server, but instead of (or in addition to) creating a CSR (which is a request for a registrar to sign your cert), you can self sign your cert - again you will get a message popping up that it was self signed, but it will work fine for testing purposes. [ If you get more sophisticated, you can actually create your own registrar for internal use and add yourself as an authority for your development machines - then you can create as many certs as you want for test purposes - without any errors (after you have added your registrar cert to your dev boxes ]
I have used StartCOM for a couple of my live boxes for a few years, with no problems. They are a good idea in as much as they force you to think and understand the whole SSL signing and trust process better then most registrars. OF-COURSE, THIS MAKES EVERYTHING HARDER TO GET WORKING INITIALLY. The disadvantage, if I recall correctly is you can't use them (ie free ones) in the name of a company, only your name. This is probably not that big a deal - unless you want something fancy like an EV Cert - in which case StartCom are, unfortunately, not the cheapest option.