Big companies have no reluctance is taking other people ideas:
- usually legal systems does not provide protection for ideas themselves,
- even if an idea is protected by intellectual protection rights (IP, as a side note please consider long procedure for application for patents, etc.), litigation is costly and long-lasting. You will have significantly smaller resources to be able to protect your idea (even if you win, by the time of the court ruling the market will be taken).
Thus, an idea is not purchasable for big companies. IP, prototype or a small company with IP is.
Even if you will have purchasable IP/prototype/company, big companies still decide which is the most beneficial: to buy or to reproduce. Most often the latter is cheaper. This is because if a larger company can, as you say, remove you from the market very easily, it probably will. If you are considering building your own company, I suggest seeing Michael Skok’s framework, especially chapter ‘Disruptive business models’ and ‘Getting Behind the Perfect Investor Pitch’. In short: if there are no technical barriers for possible competition, make a business model impossible to be replicated by competitors (real life example: Netscape market position was taken by Microsoft’s Internet Explorer because Microsoft could sell IE with every copy of Windows; Microsoft’s lost huge part of the market to the Google, because Google's business model comprised of delivering free products to consumers and profit based on advertisements).
Please also consider that creating a new market takes not shorter than 3 years from the release of the product, sometimes even 7 years (source: S.Blank, B.Dorf, The Startup Owner's Manual).
If you are interested in methods of evaluation return of investment
However, if you are interested how big companies evaluate new ideas for investment (usually for development by themselves), most important factors are soft criteria. Those depend on the project, please find some exemples of such criteria below:
- technology (in short: is it the way to go?)
- competition (the questions you’ve described can be good examples of some of questions, which companies have to answer before going into investment)
- missed opportunities (what would be the better investment, which the company could miss if it will engage in the project at hand?)
- profitability in general (how much the value of the company will increase as a result of the investment? when the project will yield return?)
- advisability and viability (in general: labour-, capital- and resource- intensity of the project)
- means - what is the funding structure (who is bringing their financial contribution? will the gathered means be sufficient?)
- in cases when capital is to be transferred to an outside entity (e.g. while investing in smaller company) - credibility of the team, which will lead development works (can they deliver? what were their previous projects? what is their commitment to the project?),
- ‘extras’ (additional cost-, income-, manufacturing- and/or public image- advantages)
There are many interesting mathematical models to calculate return on investment (ROI, ROE, Payback Period, Accounting Rate of Return, Net Present Value, Internal Rate of Return etc.) but these are only used after all the soft questions can be answered positively.
Hope it helps. I intended to make a small comment, but ended with few paragraph answer :)