From my husband:
While nesarim on their own are cheaper than bamboo mats, it’s a case of yotzei s’charo b’hefsedo. Once you have the nesarim, you can’t attach them to the sukkah (except by something that is in its natural state and is not m’kabel tumah…this excludes nails, staple gun, thumb tacks, and pretty much every type of fastener you can imagine). Even most types of string and yarn are woven and therefore m’kabel tumah. With the bamboo mats, you’re not paying for “hashgacha,” but for the attachment of the bamboo poles using non-woven string (i.e. not m’kabel tumah). (see Star-K’s website on Halachos of Schach).
Even if you had the time and energy to acquire such string and use it to attach nesarim to each other, you would have to tie them in such a way that allowed at least some light in (most wood that I have bought from lumberyards wasn’t perfectly straight, but was sufficiently even to line up with similar shaped pieces of wood in a manner that would not allow light to penetrate if the boards were placed next to each other).
Furthermore, I wonder whether the extent to which “nesarim” have been processed already makes them m’kabel tumah…