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warshawlawA guide is an opinion unless it is based on actual numbers and even then it is suspect unless the actual numbers are presented to you for review or may be readily verified from readily ascertainable sources. When a pricing decision is made for a book you can do it by averaging prices, estimating or presenting actual transacted prices.
The problem with averages is sample size and variables not reflected in the results (such as choosing a poor venue to sell, using the wrong ebay category, selling a legit item with low or no feedback, etc.). One example: several very rare Zeenut HOFers went off recently in a minor auction house for very small prices. The facts that the auctioneer (1) was not major and (2) turned off a lot of people with really high vig, would not be accounted for if the auction results are used to create a guide without explanation. Take the same cards and put them into Mastro or even on Ebay and you are looking at another zero on each sale price.
If the guide is averaging prices, just remember statistically speaking if one of your feet is on fire and the other is in a block of ice, you are overall at a comfortable temperature. I am always wary of a price on a rare item where the pricer says in essence "I lumped a bunch of results together, divided and came up with an average price." Who knows what went into that average? A great deal of statistical analysis involves weighting the results, calculating whether a deviation from the norm (i.e., a bidding fight between two rich a-holes over a common card) unduly affected the outcome, etc.
Estimating is fraught with bias on the part of the estimator unless it is based on hard numbers. If the former it is a SWAG estimate (Standard Wild-Assed Guess methodology). If the latter, why not just resort to the hard numbers?
Hard sales are always the most accurate price gauge. I use hard sales figures in my boxing card guides. If a dozen examples of a rare card transacted, I post all of them. Example: here is the 1951 Ringside Rocky Marciano listing from my book: PSA 9 $4,350.00; PSA 8 $911.00, $1,274.00; GAI 6.5 $208.50; GAI 6 $268, $185.50 PSA 6 $191.38, $207.50.
The point of this long-winded post is that if you really want to learn prices on a set like T206, you have to do your homework. Use the search function on ebay to pull up prices you need, track significant cards, and get a feel for the pricing trends. After a time, you may have seen several hundred vg commons change hands and may be comfortable saying that as of "x" date they generally retail for $15-$25. Just remember that pricing is an ongoing process. The market is constantly evolving and the feedback mechanisms available to track that evolution are getting better and better.