If they were going to buy the card for $30K anyway if you didn't discount it, yes. Even if they weren't going to buy the car without the discount, but someone else would likely come along and buy it for $30K, then still yes, you would have given away $5K.

Add on to the fact that if you sold that one car for $25K, you end up with other people who would've paid $30K but now want that same discount, so you may have given away a lot more than $5K.