I think it all comes down to what the car is worth. or maybe worth to you. If you go to sell it goes from maybe above average condition to just average and the value will be reflected.
$500 maybe not a lot for the type of repair you are talking about in only two spots, but I would look much more closely. and look inside wheel wells as well especially in the rear.
There are lots of DIYs out there for these types of repairs. I recall one from a guy who had a old 6 series. Came out ok. Clean off the rust with wire brush down to bare metal. Look inside wheel well also because i am sure there is rust in there too. Then you treat it with rust convertor and then bondo. If rust repair is handled well you can have many more rust free years from the car at least to the point that you think you are driving a rust box.