There are people who can offer more sage advice than I, hopefully they chime in for you but I'll do my best to provide some insights.
Without spending some money you're not going to get a highly accurate cost of development, imo. Start with round #'s and work to continuously sharpen them as you check-off due diligence items during your feasibility process.
There are a lot of variables to determining the cost of development.. hard costs, soft costs, financing costs, holding costs, etc. Does the city have impact fees, is there a lot of clearing and excavation work, soil conditions, engineering, are utilities convenient, plans, surveys, etc, etc.
I'd look for knowledge in these places:
- go to local real estate meet-ups and ask around. A multifamily investor may or may not be there but someone will likely know who the local players are.
- If there is a local commercial GC doing a multifamily call them up and schedule a meeting. Discuss the situation and work to get some ballpark figures on the hard costs. Contractors hate giving ballparks because there are so many variables but it can be done.
- call to your building/zoning department to learn about the process, what can be done, fees, requirements, etc. This will give you some direction.
- talk to commercial lenders. They may have some insight on #'s if they've financed a similar project and they may have some people they can connect you with. This could be beneficial on numerous levels
- contact local commercial developers. Google, pay attention to signs, etc. They can give you big picture numbers and connections if not be interested in a JV themselves which would incline them to spend some time helping determine viability and maybe even a pro-forma.
- reach out to syndicators
- good CPA's are well connected and know the numbers. Call yours and see what they can share and who they can connect you with.