I find that a setup similar to Moshi Monsters would work well for an online Tamagotchi Town. Basically, it's free to register and you get the basic features, but there's premium content that can only be accessed by members who pay a certain monthly/biannual/annual fee. Another option is similar to how some Facebook games work, where there is a separate currency that can be purchased with real money, in order to purchase premium content and access other areas where members without this extra currency could not.
The first option is good for getting a fixed amount no matter how much the member uses after their payment. The second option is good for those who want to keep purchasing more and more to access more and more. I prefer the first option... I've purchased a 1 Year Membership on Moshi Monsters for $45, which works out to be only $0.12 a day, and I find it well worth it.
... and I realise that I've kind of strayed from the question, but if TamaTown were to be brought back with a price attached to it, I would consider paying, but it really would depend on the price, and the period/method that it had to be paid. If it was something ridiculous like $10 a month, I wouldn't, but if it was in a similar range to MM, then yes, I probably would.