Valuation is decided between the investor and the entrepreneur. At the early-stage/concept stage, there is no science or formula to arrive at a valuation. Hence, you go by generally accepted benchmarks in your country, and eventually conclude a deal at what the entrepreneur and investor feel is a fair valuation.
What valuation investors may offer for the same plan depends on a variety of factors, including the quality & experience of the team, the investors view of the potential of the concept, the competition, how easy or difficult it is for other competitors to enter the market, is there any IP or competitive advantage which this team has, etc.
In all this, the quality of the team is he most important consideration for investments at the concept stage or early-stages. The same business plan, with exactly the same details could get a very different valuation for a team of college students executing it than what an experienced team would get for the same plan.