Your mom needs to make sure she's working with a decent real estate agent. If she doesn't know what she's doing, it is easy to overpay for a house.
The agent (one is plenty, give him a shot for a couple weeks, if it's not working out, "fire" him and get a new one) should be working for HER protecting HER interests - but she will not have to pay him anything. The seller pays 6% of the sales price of the house to real estate agents. 3% goes to the selling broker (the one he listed the house with) and 3% goes to the broker who brings the customer. (your mom's broker) If your mom's broker is ALSO the broker who "coincidentally" holds the listing on the house - he will make a 6% commission on the sale. (These are standard figures, they can vary a bit, but 6% is normal)
consider that for a while. On a $300,000 home purchase, each broker stands to make $9000. So they should be very motivated to help you out. It's their job to know the area, know prices, etc. If your mom gets close to buying, be sure that one of the brokers gives her a list of 'comprable sales' in the area. These will be similar houses in the same neighborhood with a list of how much they sold for. Also, if she makes an offer, it is "standard" to include that the offer is contingent on her getting financing for the house, and you can also make it contingent on the house appraising for a certain amount. That covers you twice - because a bank isn't going to give you a $300,000 loan on a house they feel is only worth $200,000.
The good news is, if you're looking at living somewhere for 5+ years, it's rare to "lose" money in real estate, but it can certainly happen. If you're not looking at it as an investment, but rather a place to live, you should do okay. Establish a comfortable monthly housing payment, and look at things +/- 10% in that price range.