I'd agree with the others so far, that the best entry cost is $0.
I've seen game systems where the base rules is a free product, but specific themed games and adventures are low cost additions to the basic game. These are often start-up products as might be produced by a smaller or unknown publisher. For a new game by a smaller publisher, I think this is a normal expectation.
The larger games produced by larger publishers can expect and get away with releasing the basic game (Core rules) as paid for hard cover, though almost always there are secondary free or lower cost entry products to get involved, like providing an SRD document with free access online to that resource. This applies to games like D&D or Pathfinder.
While there's a definite cost to any kind of game development, the needs of the Core rules is often less expensive than the cost of creating an adventure, as adventure almost always require paid-for illustrations and maps which greatly increase the costs of creation. Core rules often also include illustrations, though this is usually not necessary. Free adventures (IMO) should not be an expectation.
The only free adventure, I've ever provided for my Kaidan setting of Japanese horror, for example was the
Frozen Wind one-shot module. Because the author planned to attend Origins and run some Con games, he wrote the complete adventure at his own cost without expectation for compensation. Because I am the primary developer of the setting, and am also both an illustrator and cartographer I opted to create cover art, several pieces of interior illustration and the adventure map on my own, also without expectation of compensation. So, at this point, before that product was ever intended for public release, large portions of the development was already complete. The only needed development cost was to pay for editing, and the author and I decided (rightly or wrongly) to not use an editor to save on that expense, try to edit the document ourselves (apparently not as successfully done as we desired) and release it as a free product - a good setting entry tool to getting new fans to the setting. Had the writing, illustration and cartography not already existed - trying to pay for that with a free product is not generally possible, nor expected. It was a rare opportunity, that we took advantage of in releasing, since the work and costs involved were already generated. Also since the base product was 'paid for' and little additional expenses were included for final release, Steven Russell, the publisher, opted to have a POD printed version of the book made available as well. Keeping the free development intact, that product is at a very low price point for a print product, because the only costs to the user is the POD printing itself, and DTRPG's expected profit margin - as a free product the publisher and developers get no payment from this product.
