The value of your envelopes is very much dependent on who's buying. There are no catalogs for this type of EFO, to my knowledge.
I bid on most EFOs that come up on
ebay, if they are priced reasonably and I have a "need" for them in my collection. I just paid $6 for a wiped die. If an overinked item I wanted came along I would expect to pay $1 - $5 range. Most are pretty boring and not of interest.
In all cases, if there was something unusual about the piece, the price could go up.
Maybe some other EFO collectors can chime in to give you more data points on pricing these.