I have a TJ Magna and was wondering what is the cheapest way to get a replacement key?

Have you got at least 1 original?

Yes, but only 1, which is why i want a spare.

You can go to Mr Minit type places that can cut and code your key for $60 or so. They do refunds if it doesn't work.

Mitsi is more expensive.

Mitsubishi still have genuine keys avaliable,either at the dealer or their warehouse

You will need to purchase the key and have it cut by a locksmith(someone who can cut them) as they are a blank key.you will also need to get it programmed(by a dealer usually)

So approximate costs:

Key(genuine):$80.00
Cutting:$20-50
Programming:$60-80

Also unless it has a factory alarm,the remotes are also 80.00