you HAVE to specify in your contract how long your service contract will last. how long you must maintain it, etc. and what your rates for maintaining it will be. otherwise it's a guarantee- you'll be screwed.
I don't know about "going rates" in the "fine art" world, but what I do when I bid on jobs is basic math:
(how much time will it take)*(how much do you want to make per hour)+(how much you have to spend to accomplish it hiring, buying etc)+ (a percentage of your overhead)-(will it end up in repeat business or PR, aka discounts) = $$$job bid$$$
Chances are they want you to bid for the entire job and not for a weekly/daily/or hourly rate.
"going rates" for commercial art vary. I know 2D comp artists that make $150/hr working on commercials feature films and I know 2D comp artists that make $20/hr working on commercials and TV shows.
How much do you believe you are worth?
How much of a budget do they have?
How much are you willing to take?
Is the project cool enough where doing it is more important than the money?
I don't think your price is unreasonable at 250-300/day. Lets face it, when you are making something in Max, it's often time-based art and it takes a long time to test and troubleshoot some things thoroughly. Makes truly charging by the hour difficult.
I've had this internal battle countless times because things like interactive installations is such a niche market. I decided on a model that seems to work for me and keeps customers coming back.
For programming, I tend to put forth an hourly rate and then offer a minimum for services rendered (so even if they back out before it's done you get something), an estimated time/cost, and a maximum based on their proposal (so I don't scare them away by threatening to break the budget). For programming itself I don't charge much hourly because often I choose jobs I know I can manage in my spare time and I genuinely like the idea of repeat customers even if they can't pay much.
However, the physical setup for webcams, projectors, arduino, and things like that costs additionally 10 times as much hourly (no joke) plus equipment, expenses, and transportation. It's necessary to prevent being taken advantage of especially if the person you are dealing with doesn't know what they are doing.
Sounds like what you are doing is both hands on and programming.
The best policy is to confidently approach them with an estimate (minimum, expected, and high), and then explain the factors that will drive the price up (meetings, changes in design, unexpected on site work, etc).
Anyways, I don't mean to sound pretentious, I just know how nerve wracking the talk of money and max can be. I'm genuinely excited for you and would like to know if you get the contract. Gives the rest of us hope.