Depending upon the woods used will depend upon what cost I would think. You did a great job on this. Personally I tend to use 3 1/2 x cost for pricing as a base to work from. So let us say that you used $3 in wood, $0.50 in finish that would be by the formula 3.5 x 3.50 = $12.25 add in a bit for time ( you will never get what your time is really worth ) and that could conceivably bring it up to around $15. Some thing like your pen holder I have seen in shows for $25 or more so depending upon the show those could be a fast sell.
DW