I built a NSB/PT2 line array last summer. 49 cent midwoofers that
were modified, 16 per tower and 10 Dayton PT2 planars per tower
{$25 ea.)

While I consider this a budget line array, the plan was to make
'high end' sound and I think I achieved this as I auditioned high end store speakers and I wasn't too thrilled. The point is, while
the system is using 49 cent midwoofers and $25 tweeters, what really
gets you cost-wise is all the misc. costs that add up.

I itemized about 95% of the project on a spreadsheet counting every
screw, rags, gloves, nails, tools, glue, stain, magnets, plywood,
mdf, solid oak pieces, sandpaper, terminals, wire, solder,
chemicals, Acousta-stuf, poly batting, turntable, skidpads,
rope caulk, sanding sealer, liquid nails, bondo, etc., and the
project cost about $2k to make from scratch taking into account all
the stuff needed.

While DIY is cool, driver cost alone will not be a good judge on
project cost. Driver cost for this project was;

midwoofer = 32 * 49 cents {plus $1 shipping per driver} = $47.68
tweeter = 20 * ~$25 = $500

Driver cost = ~$547.58
Materials = ~ $1450

If I went 'cheap' on material costs, then the sound I get out of
the system would not be as good. 'Sum of all parts' is what makes it sound good including construction method and materials used.