We don't have any minimum deposit necessary to start a campaign. You can start with anything worthy of a transaction fee.
The most sane way is to start with a relatively small daily budget (any sum that is small for you) and test traffic for several days before increasing the daily budget.
Balance on your dashboard shows how much money you've got on your account at the moment.
Balance = deposits + earnings - "sent to campaigns" + "received from campaigns"
This money will be automatically spent to fund your campaign when it runs out of money. Money from the user account is sent to campaigns in little portions every 2 hours, each portion is enough to pay to ad units for 2 hours. So balance is constantly changing.
If you don't want your balance money to be sent to your campaign, you can withdraw it or leave it on the account after setting the campaign daily budget to zero (otherwise it would be sent to the campaign).
Also, we gladly offer discount coupons to our long-term partners. They reduce our fees by 50%.