We could get into a rather lengthy debate about the virtues of each price setting; however, as the original post was regarding a problem for the pricing of basic land and what that would do to the estimated cost of a deck, I stand by my recommendation. The low price would result in the lands being valued at roughly 1-2 cents each, which I would imagine is likely quite fair.
For the other points that were raised:
Wismac wrote:I think the median price is more often an accurate price of retail cost. Low tends to be 'too' low in my experience.
This would be great if TCG offered a median price, as it would be more resistant to outliers skewing the price distribution. However, as they only offer an average (which uses the arithmetic mean), you don't really have that option to run with. Further, it's possible to dig around a bit on TCG and find all those cards at, or near, their low prices. If you're buying a fair number of cards this way, it definitely can outweigh any added shipping cost. This is why I, personally, use the low value as it doesn't make any sense to me to pay $0.15 for a common or uncommon that I could easily go buy for $0.01. Heck, you could probably make money by buying up a ton of them and then trading them at the $0.15 value if others will let you (pending that someone actually wants them...).
rfioren wrote:low can get messed up, especially for smaller cards, because some people on TCGplayer make up for it with high shipping costs. I like using the mid-price, if the mid-price wasn't so wrong
Two things. First, the low price would not be affected by the shipping costs of any of the retailers, given that shipping is not included in the prices provided for any of the cards. To that end, if you dig around enough, you can find pretty decent shipping costs. A lot of the smaller retailers actually run standard mail shipping for only $0.99. Because of that I've managed to sit and shave $20 - $30 off of card orders by just digging around and looking through them. Sure, you might end up paying a touch more than the lowest possible price, but in many instances you're still not even approaching the mean value for the card.
Second, you illustrate my reasoning for using the low price for me with the last statement. Until the pricing can be sorted out, I'd suggest going with the low. Otherwise be sure to look and ensure that the value of each card in the trade isn't skewed - and sadly, a bunch of them are. Go take a look at the non-planeswalker Nicol Bolas. He's sitting at roughly $6, but in no way, shape, or form should you go by that price.
Obviously this is a preference issue, and you're welcome to go with the price setting you prefer (and that you and any trade partners agree upon), but for the purposes of addressing Wismac's land-pricing issue, the "low" price option is likely going to be your best bet.