Well. This is a totally out of the socpe of the forum Probably this belongs to the Graphic Design one.
But here are my 2 cents.
1) You do not choose Offset printing necesary for the "quality". You choose offset if you need a certain ammount of pices vs cost.
The costs of a digital print are linear. You pay double if you want two pices instead of one.
Depending on the market prices, region etc. you have the same costs on digital vs offset arround 200-300 pices. If you need one pice go for digital.
The costs of offset are some initial processes. Prepress, plates, registration, and after that the ammount of paper you want to use and each pass the machine needs to make to print.
Some providers could charge you by pice, but above the first thousand. Probably one could agree to print you just one hundred but at almost the same price than a thousand pices, minus some cost of the exceeding paper.
The prices are toooooooooootally out of the scope of any forum, becouse if your book measures one centímeter more or one less that could mean you need a diferent machine, or sheet of paper, number of pages per sheet. The coverage of the photos spend more ink, the type of paper, the varnishes, binds, etc...
In adition to that, the country, pre press control, credit, etc.