To scratch build that hypothetical SPAD for $10 you need to have the knowledge, skill, experience and tools to do so. The tools and techniques are different from more conventional planes. For a dedicated (or converted) SPADder this is no problem, but for a newbie, locating and acquiring all of the materials and tools could be more hassle than its worth.

SPAD trainer kits can be had, but their cost is no less than that of a conventional trainer. e.g. the Kombat 40 trainer is $90-

The Duraplane or Sturdy Birdy may also be an option but you will spend more time crashing than flying. My Sturdy Birdy flies a LOT faster than my plywood boxes and more often than not you'd still need to take it home to make repairs. Speaking of which, I still haven't found any of that 2 1/2 drainpipe thats supposed to be so easy to get hold of.

On the other hand, if you can find a slow flying, stable aircraft that is reasonably priced and can be easily put together correctly by a beginner then go for it, it doesn't matter what its made of.

I doubt though, that a SPAD can be made light enough to fill that criteria. The materials are just too dense. e.g. a yardstick ain't exactly a lightweight wing spar...

The current crop of .40 sized plywood boxes IMO aren't very good trainers either, they are too heavily built for their size to be slow stable performers.

Probably the best 'true' primary trainers are the vintage style aircraft, some of these are now available as ARF's but not necessarily marketed as trainers. These may look like a bag of sticks but are tougher than they look.

My latest trainer, an LT-40 ARF seems to be the exception to the rule. It is the same size as my old .60 sized trainer but is built lighter than yer average ARF thus making it suitable for a .40. I'm looking forward to seeing how this flies....