RIM is very hard to repair, because they are forged in one piece initially. It is not worthy it to repair a damaged rim, period.
$400 for 1 rim is rip off. Unless you are talking about 20''+. Maybe for 4 rims, then thats reasonable.
First, cheapest solution, you can look for a OEM rim at local JUNK YARD. Call and give them your model, year...etc.
Second, you can contact your car dealer for a new OEM, but it will not match the rest 3 rims, in terms of used and brand new. Bear in mind, dealer OEM parts almost never runs cheap.
Last, like 斑主 said, go for after-market rims. FYI, you probably want to change all 4 rims to match the look. Making sure all the specs fit your car. It seems simple, but actually pretty confusing on getting a perfect fit after-market rim.
www.tirerack.com is pretty well-known for rims also.