I don't know about the cost. I am sure it would depend on the length of time required to complete the job (time is money!). I would call at least two mobile locksmiths and get a rough estimate. Hopefully, they are competitive with one another. This would ensure that one company is not trying to screw you over.
When the locksmith arrives, I would have a third copy made and stash your new originals somewhere safe. Get a small bright florescent yellow, green, orange or pink key chain and attach it to your new key. This helps me from losing my key and makes it easier to describe to anyone helping you search for it.
IMPORTANT TIP: for anyone having a spare key made - leave the battery IN the frame when checking for proper key functioning. I made the mistake of taking just the battery with me to locksmith. When I replaced the battery in the frame, the copy did not function because of clearance issues due to frame thickness. It took two additional trips to the locksmith to get it right.
Good luck!