In need of a little advice

I've had my girl Diamond for just over a year now, she went through two heats and we kept her away from th males with no problems. On her third heat she got away from us, when we found her it was too late, she had bred with a bullmastiff down the road from us. I spoke to a lady where I get my dog food from and told her I was going to give them away (10 puppies), as they are mutts. She is a breeder (not of pit bulls), but she advised me not to give them away for free, she said I should get them all vaccines, deworming and what not and sell them. I am not trying to make money off my baby girl, but the reason for selling is so they end up in nice homes instead of homes that since they got the pup for free then they have no problem with getting rid of them for free... If I could I would keep them all so I know no harm will come to them down the road. I want them all to go to loving homes where they will be treated as well as they deserve. I was just wondering what a fair price would be for these puppies. I believe they will grow up to be beautiful dogs, but that's not the point. The lady said I should sell them for around $250-300 and get a $50 deposit to pay for their vaccines and deworming. But is that a fair price? I don't know.

In need of a little advice