Closer to 50 for 4-6 gb ddr3, depending upon the brand (they could go with 100 for 6-8 gb of ddr3), 140 for a amd phenom II black at 3.2, and thats all they'd need. chances are they won't need a motherboard, but if you ever have to do a motherboard upgrade to upgrade parts, chances are you're better off buying a used gaming pc. If you EVER have to upgrade a motherboard, it means you're using outdated or badly made tech, as the motherboard is the foundation of a computer.
You can also get a radeon HD 5770 GPU for around 85 or less.
As a computer builder, and I reccommend only Intel parts, motherboards are not interchangeable for most parts. Even AMD chips are specially made for different motherboards, they are what you call slots. Intel alone uses at least 4-5 differnt slots for their processors, and AMD uses 3-4 different common slots. So, with respect, your statement about upgrading motherboards is false. In addition, some motherboards accept ddr, ddr2, and ddr3 memory. In addition, some have slots for AGP graphics, but most newer boards have PCIe-16. The combinations and changes go on and on.
Also, you are NEVER
better off buying a used gaming pc unless you know the person that is selling it to you, or it is warranted. Gaming pc's tend to be abused and overclocked. This causes the parts to wear out faster.
As far as the memory goes, I did a quick search and found that the most popular speed of DDR3 runs around 50-120 for 4gb, with the average around 70. Again, this is the most abundant speed available, therefore, the most popular.
Clock speed of an AMD vs Intel at the moment is not matched appropriately. A 3.2gb intel is faster than that of a 3.2 gb AMD. This may change in the future, but for now, it is just not true.
Also, INTEL is better for gaming as is ATI graphics. (I know, I use NVIDIA, but the research shows that ATI is better at this time)
I know that this has been said before elsewhere, but is worth bringing back to the forefront. But remember, when it comes to gaming computers, cheaper is NEVER better. You will always get what you pay for.