How much is it going to cost you...
Offering free games is workable because players who enter their own orders online and whose game reports don't need to be printed and posted cost very little extra in a game that's already running. The costs of design, development, maintenance, production, presentation, advertising and setting-up still remain, but those are mostly overheads which don't increase much when extra players are added.
Given all the costs that still do apply, a game wouldn't be viable if it didn't have people playing (and paying) normally. They're the ones who are paying for all those development costs. And it wouldn't be worth offering it for free except for the hope that if you like the game and find it works for you as a freemailer, then you might upgrade or have a go at playing a different game later. That's also why it's good for the paying players - they want more people to play against (or with, as the case may be).
If you play online and no longer qualify for the freemailer offer then you can take the pay-as-you-go option of paying turnfees (see below) or you can pay a simple annual membership instead. This costs £24.50 per year in the UK and $74.50 per year in Australia (US $48.50, Euros 44.50). That includes an extra freemailer startup each year (in another league or another game).
Memberships don't renewal automatically. There's no re-billing. Renewals only happen if you log on and push the buttons yourself.
In postal games you pay per-turn, in advance, and that's still allowed for online players if you prefer to pay as you go. The number of turns you've paid for (but not yet used) is given at the top of each game report. One turn credit is deducted for each turn you play. Turns that have not been paid for will be played but not sent. Your commitment lasts only as long as your turn credits, and you can drop out anytine.
In all our games we can keep track of tenths of turn credits ("part" credits) as well as whole turn credits. This allows us to make extra charges if needed. Our rule is that we make no extra charges for things that are relevant to how the game is played, but only things relating to extra costs (eg. fax costs, extra changes of orders, extra copies of game reports). All our turnfees are "flat", which is to say they don't change during play (except when we change our prices generally). You will not be asked to pay for more a game because you're doing better than the next guy.
Turnfees for pay-per-turn players in Dark Age in the UK are £10.00 for four, £20.00 for ten or £36.00 for twenty. There are more discounts if you play in more than one game and a bonus for introducing new players.
Play-by-mail games are often the best value you can get, because if you don't like a game you stop playing. And stop paying. No-one can make money from putting rubbish in a pretty box when it's pay-as-you-play.