8" / 10 " refractors DIY or not are very much a specialty Instrument
and can get a little pricey even before you mount them on something
http://apmamerica.com/index.php?main...products_id=38
A 12 " dob is a great step up from the 5" and very doable with some pretty basic mechanical skills and is very much made to your specs .
Yes its not always cheaper to build , but it can be fairly competitive
if you do a lot more yourself .
My 12" truss probably ran at about $1300 finished and waiting for a primary mirror and argo navis. The break down went.
second hand p/star secondary $100
new pstar spider to suit $240
moonlite connecters $300
10000 tic encoders $180
primary cell $35
encoder mounting gear $10
kineoptics focuser $110
ply ac pine $150
truss poles $25
paint ,screws ,band aids $80
bits n pieces the rest
The spider and connecters @ $540 could sure save some dollars
off this amount if you wanted to do something yourself.. On a dollar basis I'm pretty sure you could custom build a scope for not a great deal more than the $ 1200 or so the collapsable ones sell for includeing a
cheap asian primary $650.
best wishes to you if you decide to go down this most rewarding road