USD$8 for PCB assembly by machine? That's impossible! I do PCB and PCBA for several years, it always have the minimum consumption for SMT by machine since the the booting fee is there. For your reference, zfpcba is 196usd, pcbcart is 203.2usd, 7pcb is 200.04usd. The quality of PCBA boards by machine is high and it's beautiful. If your project is just a simple prototype and not require high quality, I suggest you solder by hand.