I got the idea from an instructable I spotted a while ago, forget the url but you could google it.

A Mont Blanc rollerball is a very nice pen, it writes smoothly and the body is nice and solid and has a good heft to it, the downside is that they go for about $200 give or take. If you're like me, pens come and go for some strange unknown reason. Whether it is lost or "borrowed" from my desk, they just don't seem to last. I think the most I've spent on a pen for myself was about $80, I have no idea how long that one lasted but it wasn't worth the $80 for the amount of time I had it in my possession.

Anyway, this rocket scientist figured out that a Mont Blanc rollerball refill (about $12 for 2 of them) is almost exactly the same as the G2 refill and in fact, that the only significant difference was the length. He also discovered that if you hack off a few millimeters off the end of a Mont Blanc refill, it fits perfectly in a G2 giving you the writing smoothness of a $200 Mont Blanc in a $5 pen. Now the G2 is no Mont Blanc, but it's a pretty decent pen ergo-wise.

So I tried it and it works, here you go.