allow avatars and sigs for subscribed members. I don't have good suggestions for subscription prices without knowing what you're paying for the server.
One possibility you could consider, if you need to bring others into the game, is to use a kind of invite system. You know, say we tell someone about the game and bring them in, and when they make an account they refer to the inviter's account, and the inviters account gets something like in-game money or whatnot. Or, do something like what Eve Online does, if you bring in someone that subsequently subscribes, you get a free month of subscription time. It helps increase your member base, which in turn lets you lower the individual subscription prices to something like 2 dollars a month or whatnot.
Another possibility, in combination with a lower subscription price and the bronze account mini-map suggestion, would be a kind of donation meter. you figure out the amount of money you'd need to justify opening another large map, and set up a donation meter showing how much people have donated. Once you hit the designated amount, the new map opens up for new subscribers to join, as well as anyone who donated. With the subscriptions itself, make it so that one subscribed account=access to one large map, then donating to open the new map also allows you to start a country to the new map.
So, a summary.
Free Account=access to small maps without, say, access to nukes, and with a lot of banners. If they are made and put in that they were referred to by a subscribed member (watching for obvious abuses, i.e. IP sharing, same email, etc.), the member gets $50k or something like that as a bonus. If the free account gets turned into a subscribed account, the referring member gets a free month of subscription time. Encourages bringing friends into the game, which is a plus for you, Johnny, and encourages subscribing and encouraging subscribing, more plusses, while still giving people the ability to play the basic game for free as a trial. also, give them access to a reduced forum area.
Small maps=no nukes, no alliances, banners, and instead of deleting the free accounts after 30 days, just have the maps only last around 30 days. After 30 days, reset the small maps. Adjust the number of small maps based on the number of free accounts; 1 map per 30 players or so. Purge the free accounts after 30 days of inactivity.
Subscribed members: $2-3/month (as a suggestion, once again i can't give a good one without knowing your costs and desired profit, etc.), gets access to play the large maps that lasts however long it takes. Allows a country on one map per subscribed account, however give subscribed members the option (through a subscriber-only section of the side, for example) to donate to the site to fund another large map (minimum donation $5, to prevent abuse). People who donate get access to the new map in addition to the one they already have a country on. Allow the option to delete countries to join the new map. Subscibers are also the only ones who get access to all the site, including map-specific forums for diplomacy/trolling, discussion forums, etc. Large maps only end when one alliance/country takes over the entire map, at which point the account(s) that win get a cool badge on their account that will show on the forum as well as in their user information; e-peen material, basically. Maybe some other tangible benefits as well for victory, like an extra $10k the next time they start a country on a map, etc.
In the meantime, if money is becoming an issue, I would strongly advise opening a route for donations now so you aren't too far in the red before the game is finished. I know it looks scary being in the red for your investment, but that's just what the barrier to entry is; if you jack up your prices right away to get into the green, you're going to drive people away. Everyone is a fickle little bastard on the internet, its a fact of life. If you keep the prices low and convince others to do the advertisement work for you to draw in members, you will be more likely to make back your investment and will make it back quicker by having possibly twice as many people paying small amounts all at once. Just think, half a dozen people chucking $15/month only nets you like $90/month, having 40+ people paying $3/month nets you at least $120/month, with the potential to draw in a lot more people that will also pay the $3/month. If the game is developed further, which it doubtlessly will be i'm sure, the more likely people will be wanting to pay to play it.