$700 for a two flat silver coating? Please don't call me if you think that's expensive. A good quality silver coat such as Karnak 28 is $68 per 5 gallon bucket plus tax and delivery. I wouldn't use the stuff at home cheapo unless you want to repat this process every 2 years. In short it won't last as long as a good quality product. It'll look the same on day one but in year 3 you'll see a huge difference. The Karnak #28 is applied at approximately .75 gallons per 100 square feet. That's 3 5 gallon buckets. That's $250 plus labor.
Don't forget the repair/prep work. The coating is absolutely useless if the roof is in dis-repair. The purpose of a silver coat is NOT to fix the roof. It is nothing more than sun screen. Can it extend the life of a roof? Yes and no. The UV coating will prevent the sun from damaging the asphalt. The sun will dry out the natural oils causing pre-mature failure. Will silver coat add 5 years to an existing roof? Only with proper prep, meaning patching anything that may be wrong and fixing any split seams. The silver coat won't do anything to correct this.
There are coating products on the market such as Rm Lucas #6000 Universal. It'll cost ALOT more than $700 though, closer to $7,000 and yes these elastomeric roof coatings will extend the life of a roof, given the prep work is properly performed. These products are applied at anywhere from 2-3 gallons per 100 sq ft. BIG difference with silver coat.
Learn more about roof coatings and your flat roofing options. But please don't call me if low price will be your deciding factor.
One more thing about coatings. Your roof has to be in reasonably decent condition to have an elastomeric roof coating applied. I look at alot of roofs in and around chicago where they were sold a magic cure in a can, and it didn't work. Then the coating gets blamed, but the roof never should have been coated in the first place. If there is trapped moisture in the roof assembly, don't coat it. If there is seperation of the layers which has caused buckling and ridging, don't waste your money coating it. I was at a repair estimate last week, their roof NEEDS to be torn off, it's that bad, but some a-hole promised them it could be fixed with a coating. Well I won't be involved with that circus. They like all my customers will get a price to do it right or nothing at all. In some situations a coating may be OK, but not always.