I think you'll find the money that you save could cost you money in the long run.
Speaker wire is usable, but for a variety of reasons (gauge, insulation type, metal purity, etc) it really is not ideal for this type of thing. Wire nuts are a bad idea. They are meant for solid core wiring that is used in building electrical, not for automotive. The vibration inside the vehicle will eventually cause the wire nuts to come loose. If you need to splice into the factory wiring, use automotive wire taps if you don't want to solder connections.
If you are going to tap into the low beams, I would suggest creating a simple harness with a relay so you don't overload the factory electrical. Tapping into an existing circuit like the low beams can be done, but its really not a good idea. We get calls all the time from people that wired the lights themselves to save money and ended up running into trouble so you have to be very careful.
I am not at all saying it cannot be done DIY, but the person doing it needs to understand the way these newer electrical systems operate. Things just aren't as easy as they used to be...
For what its worth, I just looked and we have one refurbished 05-09 harness for $49 bucks on our website. Still caries the full warranty and new these cost $100. All you would need then is an OEM headlight/ fog light switch which is $70 from us.
Hope this helps!