I have for sale a custom built (at professional audio shop) sub enclosure with a morel 10" ultimo sc sub, along with a Kenwood x1d amplifier to power the beast.
The original cost was £500 for the sub and £500 for enclosure. And I paid ~£280 for the amplifier at time of purchase. I have pics of it in an m3. Currently housed in an e92 335i.
I am on my mobile now so can't directly upload pictures but below are some links to pictures*which were taken in the past. The 335i it is currently fitted in does not have the ski hatch but it is very easy to cut one in through the back panel and just put acoustic cloth on afterwards to look oem.
Looking for ~500 but open to offers.