I have the vanilla game on disc(came with the LE of Heroes V) and can already tell the game is good from what I played but I was wondering how much the expansions add and are they worth $10?

New campaigns, maps, heroes, rebalancing of existing units. A few new cutscenes. The main advantage of the GOG version is no need for a disc and simple installation.

Also some new units and IIRC the expansion added the artifact sets.

Disc checks don't bother me. That sounds like quite a bit of stuff I'll be missing without the expansions though, so I will probably get it here later.

One of the expansions added the random map generator, which is my favorite part of the game.

IMHO RMG works really poor, I prefer to look for maps on the internet.

However, apart from what said above, there are also small changes in units. 3DO discovered that castle Castle (sic!) is a bit too strong and it changed Angels and Archangels cost - the former used to cost 3000 gold, now they need an additional gem, the latter used to cost 5000 gold, now they need additional 3 gems. It makes a huge difference!

There are also minor things like being able to buy non-upgraded units even after creating a building for upgraded ones, being able to re-visit a place by hitting space, getting damaged by staying in a moat during the siege, being able to leave guards in mines etc.

And mods like Heroes 3 HD (it's really nice to play in 1440x900 instead of default 800x600!) work only with Shadow of Death or Heroes III Gold/Complete (the latter available here) so even this is worth buying.

BTW, this is the first time I buy this game... I've had a standalone SoD and the Gold version before but the latter one lacks some SoD improvements like changing Angels/Archangels cost so I figured I should re-buy it anyway... Unfortunately Ubisoft have kept selling the Gold version instead of the Complete one recently...