A disc caliper (with lever / as kit, if doing hydraulic), rotor, replacement wheel, replacement fork, and if you're not doing the installations yourself then a fairly decent labour charge to go with it. That's a fair bit of labour for a shop to completely remove and replace these components.

This is also presuming that you can find a fork with the appropriate dimensions to replace the original or provide the ride you like, otherwise the bicycle's riding characteristics can be severely changed and you might not enjoy the end result (important!)

I second the notion that jumping early and ending up having to chase parts for upgrades won't be economical. The Elite Disc does have better parts in other areas of the bike as well (better rear derailleur, adjustable stem with o/s bars, higher end crankset), and it offers more value for your money.