All numbers are approximate:
If buying a house takes 25% of earnings for 20 years, the house costs five years worth of annual earnings. If the players wish to live with the artisans, the going price should be five years of artisan wages. According to the DMG, an artisan would earn about 1000 sp/year, so the house would be approximately 500gp.
A workshop/studio with living quarters would probably be between 450 and 800 square feet , so a 10'x10' area might be worth between 60gp and 115gp. Each additional floor might add 50% to the cost. A basement would add 100% to the cost.
Example: The player characters want a 40'x40' 3-story complex with basement. The cheapest they should be able to acquire one might be 500x2x300%=3,000gp. If they want it built to their own specifications it should be at least double that, plus the cost of any special features like traps and fortifications. Luxury architectural features (a temple inside for example) could easily double the cost as well. So it might be 12,000gp for a beautiful, luxurious, but unfortified complex in the artisans' quarter.
Setting up shop with the artisans would probably require that the guilds be paid off, to the tune of 10%--1,200gp in the above example. Maintenance might require 1% per year in building supplies and another 1% in wages for the tradesmen, and the adventurers would be expected to pay both guild dues and city taxes, since they are making this place their official residence. These dues and taxes might be fixed or they might be income-related, but the neighbours are possibly paying 10% to guild, 10% to city, and 10% to monarch, for a total of about 300sp per craftsman per year. If the adventurers and their henchmen are considered to be about 20 artisans, that would be a collective tax of 600gp/year.
Hope these figures are helpful. You may have to rework them a bit but the example might save you a bit of work.