Also, you need to be more specific with what, exactly, you would be happy with receiving.
When you say “concept,” many things can go into that:
- overview/feel color artwork to describe the look of the game
- color and/or line artwork of various entities/units in the game
- a treatment on what the game player will find fun and unique about the game
- description of the behavior of the different units/entities
- description of the capabilities of the player in the environment
- if there is character progression, description of how this works
- if there is a game story, a draft of the game story points
- if there are particular rules or rule sets that make sense, a draft of the rule set (“tech tree,” for example)
- a blocked-out, blue-and-gray, example “slice” of what gameplay might be like as a prototype
Each of these things will/can be created by a different person, although for them to all work together, the group that creates it must be in communication.
The cost for each of these line items may also vary by up to a factor 10 based on who does it, and another factor of 10 based on how much work you want put into it – something someone cranks out in a day is different from something that has weeks of careful iteration.
Also, another factor of 10 comes in based on how detailed you want the game to be – how many environments, how many props, how many characters, how many enemies, …
And another factor of 10 based on how much or little of the above list you want.
So, this pre-production stuff could roughly cost you something between $100 and $1,000,000 depending on all of the above.
So, if you can actually be more specific about what “successful delivery” would look like to you, in concrete terms – how many full-color artwork pieces, how many pages of story, how many different enemies, environments, rule sets, … – that would make it possible to focus the discussion better.