You won't want to replace your used parts with other used parts for this application. Everything you list are wear items, and will be worn in any engine.
The valves are not re-grindable (stellite coated), so used valves just won't cut it.
Asking for a used timing chain would be the same as asking for a used drive chain. It's gonna be stretched, and it will wreck your sprockets.
Nobody makes aftermarket items which are cheaper than the stock stuff (unfortunately, it's not like car parts where there are many sources).
Aftermarket parts will offer more performance, but cost more than the stock parts.
Megacycle and Web both do cam grinds for the DR, but they require a good core to do the hardweld on. If your core is bad, you'd end up paying the core charge (cost of a new cam from Suzuki) PLUS the cost of grinding it.
Aftermarket valves may be available from Vesrah, but after trying them a few times, I've found the few $$ saved isn't worth it. The stock ones are better quality.
If you have only priced these items at your local Suzuki dealer, try one of the national dealers, you might be pleasantly surprised.