No matter what the initial fee is, you will not be required to come up with additional money until the first Office Action comes from the USPTO. This is not likely to occur for a year or longer, so you will have time to accumulate the money to pay for the response to that Office Action.
Your attorney’s or agent’s responses to the Office Actions will typically cost $2,500 for a response to the first Office Action and about $1,500 for a response to the second Office Action (if a second one is necessary). At this writing, the utility patent issue fee is $700 and your attorney or agent may add in his own fee for handling. Utility patents will have maintenance fees payable in six month windows which open at of 3 , 7 , and 11 years after the date the patent is granted, ranging from $450-$1,900 at this writing (based on which maintenance fee is being paid). Again, these are the USPTO fees and your attorney may charge a bit more for handling the payment of maintenance fees for you.