First: You do not require sound proof rooms for conversion into digital format. All you need is a good cassette player and a PC with a good sound card.
Second: Here the time required would be 90X60 minutes or 90 hours just for playing the cassettes, then you have to check the recordings too. So double or 180 hours add another 10 percent for changing of cassette etc. So a project of around 200 hours. Working at $1 per hour, I would settle for $200 for the project. And working 8 hours per day you will need 25 days to finish the project, if someone doesn't want to check the finished product thoroughly then you sure can do it in less time.
I would give you the best possible recordings and the MP3 format at 128bit or higher as per your wish. All the songs will be duly named and tagged with the singer's details as per the cassette.
The finished digital content will then be burned on CDs and sent to you along with the finished casettes.