Actually, the Pilot Varsity isn't truly disposable, because it is so easy to refill.
I have one floating around here somewhere, filled with invisible ink. Unfortunately, I can't find it.
Easy... It's kinda a pain in the ***. I'd say it's cheaper in the long run to plomp down 15 bucks for a Pilot Metropolitan and use the squeeze convertor for multi colored inks. One squeeze lasts me a few weeks and I write a lot with it. Though it's not as user friendly as Lamy's converter.