Depends. Sound techniques aren't my forte. After I buy a sub/s I would pay someone to install them and if need be also have a box, too. The cost for install doesn't factor in the price of the subs
1. BUDGET - $300
2. SQL
3. A trunk space worth of space. (Not a big factor)
Also, would a pair adding up to $300 be better than one sub costing $300? What type of boxing/brand/etc. do you recommend?