Hi Dave,
Seeing no one else has responded to you, I will give it a go. That said, I am not a logging expert and wouldn't be able to answer your question, even with all the data, but I can think of some reasons why you may be getting lower bids than you thought you would.
The main reason is the size of the parcel to be logged and the number of logs to be had. I do know that in the logging world, your parcel would be considered very small, probably borderline as to whether they can actually make any money.
Logging companies these days work almost exclusively with large, heavy equipment, it's not a man and his chainsaw anymore. It takes a lot of time, effort and money to move heavy equipment into a forest. And with an 18 acre parcel, you are probably talking about 2-3 days of cutting, certainly no more than a week. Which means the loggers have to recoup the money to move men and equipment in a one week window. In larger parcels, the loggers may spend weeks or even months in one parcel, thus spreading out the cost over a much longer time, and far more logs. Personally, I think that is the biggest reason for the low bids. The overhead costs of cutting a small parcel is probably virtually the same as a much larger parcel, they can't eat that cost or they will lose money on the cut.
You also say there are 20 species and 271 trees. That means there are only about 13 trees per species. So they have to identify and separate each species and keep them separate throughout the process. Again, adding to the overhead cost. If it were two species, with over 100 trees per species, it would be a lot simpler for them.
I'm not trying to protect the logging companies, I have no stake in doing that. But I can understand that the value of your trees is small compared to the overhead costs of cutting your parcel. You are in affect, being charged the cost of cutting (which I believe to be fair) and since the return on your parcel will be small compared to the value, the logging companies can't afford to pay more.
What I would do, if it were mine, I would call the forester and ask for their input and specifically ask them for the "best, most honest loggers" in your area. They should know that, or at least have an opinion.
If you have three or more bids on the parcel and all are about the same, I think you already have your answer. I don't think they are trying to "screw" you, I think the incrementally higher cost to cut lowers the over-all value. Remember, loggers need to make a profit too!