I would say your cost of materials will run between 2 and 3 cents a foot. My material cost is 2.13 cents a sq. ft. but I am in the north.
If you go with the 100 gallon unit to start most of the guys with smaller units us a product called Jet Spray which is made by Profile since the tank opening is a little smaller on the 100 gallon units than on bigger units and jet spray is basically flakes that you can just poor in quickly. It does take just a little longer to mix than paper mulch which would be your other alternative. One bag of Jet Spray is about perfect for a 100 gallon unit and that should run you $ 13.00-15.00 a bag. A 100 gallon unit covers 1350 sq. ft so you would have about one cent in mulch per square foot. Seed costs are going to depend on the type of seed you are using. With Bermuda you would seed at about 2 pounds per 1000 so about 2 1/2 pounds per tank, fescue about 10 pounds per tank, Rye about the same. Bahia a little less. You can check seed priced in your area and figure the cost for that. Fertilizer is not all that expensive nor is tackifer which are the two other things you would need to add. I have heard the cost per square foot from a lot of people in a lot of places and what I quoted should be close. The highest I have heard was 4 cents a foot but those were guys adding a lot of stuff most don't add like organic biostimulants etc.
A nurse tank is a good way to go. It will let you do the jobs you are going to get pretty quick. I can recall talking to one guy who had a 150 gallon unit with a 150 gallon nurse tank and he told me that he could seed faster than his competitor with a high priced 500 gallon unit but no nurse tank.
I have seen a number of guys start in with 100 gallon units and do just fine. Usually after they see how much hydroseeding business is really out there they end up trading it in for a bigger machine but it isn't a bad way to start. One that I know of who did that is a lawnsite member MarksTLC. He started with a 100 ran it two years, traded that for a 300 ran that for two years and last year traded for a 500.
Getting back to mulch again, Paper mulch is another way to go. It is just a bit harder to add but cheaper and mixes better and covers better with less material. One good mulch is Central fiber which is what I use and they have three plants one of which is in Tyler Texas. They should have a dealer close to you. Paper mulch should run about $ 9-10 bucks a bale and you might only need about 3/4 of a bale per load. so you would have a mulch cost of around a half cent a sq. ft. You should be able to do the math with what I told you to figure your own cost per sq. ft.
One thing you might want to do also if you get into hydroseeding is to join the International Association of Hydroseeding Professionals (IAHP), It is only $ 100 bucks and they list you on a "find a hydroseeder" web site that gets a lot of action. One good job would pay for your membership many times over and I have seen guys who get as many as 75 jobs a year from that listing. Some get less but it is a really cheap investment in your business. I get 3-4 calls per day myself from my listing and probably got 40-50 jobs from it.