Not sure I follow, I have to go for bigger towers for it to pay off?
No, what I'm saying is that you're probably not going to be building anything to compete with commercial speakers that you can buy for between $50-200. Mainly because, at that price range, a big company can manufacture higher quality parts in bulk for cheaper than you can buy individual parts for DIY. This is also why I said it depends on what you consider cheaper. If you're going to go the kit route, you'll probably spend $2-400 on a parts kit, then either lumber or a flat pack/prebuilt cabinet, finishing materials, and whatever else you need to bring the project to completion. Depending on how elaborate you get with your finishing you could be looking at tacking on an extra $50-250 onto the kit price for a pair of bookshelves. So you'll end up with a $300+ pair of DIY bookshelves that will likely be able to compete with commercial speakers in the $4-700+ range (your ears are the qualifier here).
The other point I'm making is that the more expensive kit you get, usually the greater value you'll see in terms of raw performance. Most manufacturers aren't putting $1000+ worth of drivers and crossover parts into a $1500 speaker. So if you buy a really good parts kit for $1000 then put $500 into building some really nice cabs (depending on your ears) you'll likely be comparing your $1500 speakers to much much more expensive commercial offerings.
This isn't to say that a less expensive kit will be less valuable. I built the Mandolin kit designed by Jeff Bagby which was around $350 for the parts kit, did a simple/cheap build for around $50 in some maple trim, leftover Maple veneered ply, and poly. To my ears, I have no problem putting them up against nearly any bookshelves I've heard at any price point. They're not the be all end all, but they're very good and for the money I put into them it was well worth it since I would've had to spend a lot more on a pair of commercial bookshelves to get the same quality.
Hope that makes sense.
