Have you checked out the Budget and Avis forums here on FlyerTalk? You could find a discount code (not coupon code, you're getting 3K miles with the coupon code) that could wipe out that extra $20 over the Enterprise price.

Avis AWD: Avis Discount Codes
Budget BCD: Valid BCD Codes

The best way to use these two threads is to start at the last page and work your way back.

Keep in mind that rarely do Avis and Budget require one to use a specific AWD or BCD code in order to use a coupon code for miles. So don't be misled by the ad copy that encourages you to use AA's corp code when using a miles-earning coupon code. Almost always the T&C state "Offer may not be used with any other coupon, promotion or offer except your AWD discount."

I'm guessing that you are going to use the MUAA022 Avis coupon code . I've used this coupon with a Costco AWD (A108364) for a really great rate. Miles posted with no problems.

Why pay $20 for 3K miles when you can possibly pay $0

Other AWD codes you might want to try are A415900 (BJ's) and B021615 (Wal-Mart Shoppers).

For the record there is 3K bonus promo for US (3-day min) right now as well as a 3K bonus for CO miles (5-day min). More here.