Left my commuter outside for 3 weeks and the chain has rusted solid. Need to get the bike on the road again but do I need to buy a whole new drive train because I need to replace the chain?
how much am I going to have to fork out for falling off the rails?

If you’re sure the chain’s dead get a new one, stick it on, see if it goes. If you totally wear out a chain you’ll often need new sprockets and cassette but, if this one was just part worn then died from neglect there’s a decent chance a new one will do it with no other parts.

I’d go for the good rub down of the chain with a brillo pad then some oil and it’ll all be hunky dory again.