What would be some good and affordable mountain bikes for a beginner?
Forget about chain store big box stuff  - cheap, nasty and comes with no support, They typically ride like pigs, and are usually exceptional heavy.  You get exactly what you pay for  - I won't put by 10 year old on one. 

Cheapest is second hand, next cheapest is last years model on 50% mark down. Either way, go visit the LBS's around you and find one that has lots of kids bikes - its a sign they know about tight budgets. They also tend to be keen on the returning customers (kids grow out of bikes really quickly), so tend to give good service despite you not putting downs $1000's on a flash bike.

Do not pay sticker price. negotiate.... You will also need things like pump, helmet, spare tube etc. The LBS will also make sure you get the right sized bike. 

If they are still out of your price, ask if they will check a second hand bike if you bring one in. A few dollars spent doing this and you will end up with a better bike for less money than a chain store bike.