Well I have two XXX 15's (new) and two PG Tantrum 1200.1's (bought used), and I have about $1600 in those 4 items. Halve that ($800) to get an idea what you could do.
The box costs can vary anywhere from $25 (cost of a sheet of MDF) if you build a simple system yourself, to over a grand I imagine if you payed someone to build some elaborate enclosure, depending on labor costs, how complex, fiberglass vs wood, etc. Point is its real hard to estimate enclosure cost, unless you build it yourself.
edit: IMO I would rather spend more and get the better sub (not saying the XXX necessarily is, although -IMO- it clearly is, so dont ***** lol) and spend less to get a cheaper amp, if that's what it takes to stay within budget. IOW imo if you want a XXX, you should look at a cheaper amp than the nine.1 if both dont fit your budget.