On a 7000 point system, I recommend going for quotes and investigating each of their offerings. Limit the DCS/PLC vendors to 3, possibly 4, to make your review easier for you.
Costs on a 7000 point system can differ up to 40%, so it's worth taking a look at multiple vendors. You also need to have intimate conversations with each vendor to understand their systems offerings and capabilities. A 10-20% cheaper vendor may not have all of the tools that fit your plant and personnel. For a 20year investment, this 10-20% up front savings may cost you more in the long run.
Using US$1000 per I/O point when designing a new system.... Add the vendors costs and your internall engineering, and it will come out to be about $1000/point. So, for 7000 points, it'll be US$6-8MM.
A 20-40% difference in cost between vendors is money you can't afford to give up without a comparative bid!
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