I don't think you can get much to mine GLD or any other scrypt based currency in $100.
I've a intel i7 2600k which gives 45kh/s with cpuminer. I added a XFX AMD Radeon 6770 to it and the card only produces 125kh/s. This card costed me about $105.
Looking at this, u may opt for a cheap cpu with AMD 5450. thats the cheapest one in the market. But It'll not be able to deliver more than 70-80 kh/s. With that speed to mine a block of GLD at present difficulty u may need to wait for a month....
