I'm wondering about how much it would cost and what conversions I would need to make to run it in a 99 7.3l SD. Is it even worth the money??

 

You don't need a conversion to run biodiesel in your '99 PSD. It's compatible as it is. No additional costs or modifications necessary.
Just fuel up and run it.

 

Are you going to brew your own or buy it retail? If the later, it all depends on how you look at it. When I can find it...and it's not 50 cents more a gallon...I burn B20 through my '04; that's 20% less foreign oil that I'm burning. Someday I'd like to homebrew...but it's not a safe option for me right now. As far if your truck will burn it...or be ok. It will burn it all day & night long. The only problem you might have is when the biodiesel cleans your fuel lines out. You might clog a couple of filters at first, but after that...all will be well. That, and cold weather gelling...depending on where you live.