That's like asking how much to buy a house. You'd need to know the market, how nice of a product (Class A vs Class D), how much SF per unit, etc.
To give you actual numbers, you can buy at $50k per unit, and all the way up to $200k+ per unit.
multi family investing books are a great place to start (really excellent resources available through BP press). Price is really determined through NOI and is can be adjusted by different factors..
@Landon Belk this is a question like "How long is a piece of string"....
Its about locations and type Cap rates, NOI and then sometime knowing how much you should pay per units.
Real I suggest you get educated before buying anything.
I recommend thinking about this differently. Rather than asking "how much can I expect to pay for a 200 unit apartment complex," ask yourself "how much capital am a capable of accessing?" This can be your own money or passive investor money. Once you know how much capital you can raise, you can calculate how much real estate you can buy. For example, if you can raise/have access to $1 million, you can buy real estate up to ~ $3.3 million (expect to pay 35% of the purchase price).
In regards to cash flow, that depends on your goals. Determine what ongoing return you want and find the best strategy that aligns with that goal.