I have an 01 BMW X5. I am using the stock headunit (non-DSP) and replaced all the factory speakers with component Focal 165 KFs (80W nominal power) in the front doors and coaxial Focal 165 CV1 (70W nominal power) in the rear doors. An Alpine PDX 4.100 amp is powering the front speakers. The system sounds great now but lacks the depth that only a true sub can bring.

I am trying to decide between subs to fit in the hidden right rear pocket. My amp will depend on the sub. I plan on using either an Alpine M450 (220W) or Alpine M650 (400W).

Here are my options:
1. BSW sub box. Uses an 8" Alumipro Alusonic EX 8" sub with 150W RMS. Around $650 with amp. This forum just raves about this option.
2. JL Stealth box uses the 10" 10W1v2 sub with 150W RMS. Around $600 with amp. I am a bit skeptical of the JL Audio sub SQ.

OR one of these custom options:
3. Focal 21V2 8" (250W) for $300 (recommended .42 c.ft sealed enclosure) in a custom box. Around $450 with the amp + custom box. I prefer this option as it keeps the speakers consistent. Is it reasonable to get .42 c.ft out of that space?
4. Alusonic 8" in a custom box ($150 for the sub) (recommended minimum .35 c.ft sealed enclosure .5c.ft optimum) Around $300 with amp + custom box.
5. RE Audio RE 8" D4 (recommended .3 c.f.t to .35 c.ft optimum sealed enclosure) ($100 for the sub)

I want to spend the least amount of these options, retain sound quality but don't know what a custom box would cost. How much do most shops charge for a custom box in that area?