4gb micro sd $6+sh, woot
 

Cube Memory 4GB Micro SD with Adapter $5.99, $6 at woot, thus it's $5 shipping for 1 or for 3.

http://www.woot.com/

Re: 4gb micro sd $6+sh, woot
 

Noteworthy points:

They are SDHC (not that the tablets care with any recent OS, but some devices do).
They are class 4 (4 MB/s minimum for sustained writing on a fresh card).
They have only the full-size SD adapter; no miniSD adapter for N810s.