A boiler operating 24 Hr now produces 600 psi steam. If I need to use the same boiler for 200 psi steam only, how much minimum cost ( a factor like percent)in fuel cost, I can save?
Layman's Term please.

No thumb rule for it and it all depends upon the boiler capacity, drum and pipe volume and load. In other words, if you have a boiler with a capacity of 1 ton/hr and you are constantly using 1 ton/hr, right from the start up, then your boiler can't develop any pressure and thus it is a wrong selection.

Secondly, you are not supposed to run your boiler at a reduced pressure than it is designed for. There will be steam quality problems and also energy inefficiency(I am not speaking about energy consumption).

The cost difference in producing 600 psi steam vs. 200 psi steam is all in the up front capital.  Your fuel usage is a function of the demand, not the supply pressure.  If the load requires about 200 psi dP, then there is a roughly fixed energy requirement to recover that dP, whether you are going from 0 to 200 or 400 to 600 psig.

You might get some marginal saving because you might be able to run lower flue gas temps, depending upon whether you've got an economizer and/or air heater. My experience with this kind of exercise is that it often looks FAR better than it really is, because of steam metering that is not pressure compensated. If it doesn't get recalibrated to the new, lower pressure, the meter reads high. There will also likely be steam quality issues at the lower pressure. Spirax Sarco has (or had) a video shot from inside the steam space of an operating boiler. You can actually watch the "fog" form inside the boiler when the pressure gets reduced from it's design operating pressure. You can also run into problems with system components (piping, control valves, traps, etc.) being sized for the current pressure, and not being big enough for the same #/hr at the lower pressure.

I can't think of too many plants that have tried this, that have actually had the headaches outweight the benefits. The ones that were convinced that they had acheived the greatest efficiency increases were typically being fooled by their steam meters.

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