1. I have a voltage range of 40V-100Vdc & have to make 5vdc from it and power a microcontroller with it. Max current is 700ma.
2. Since have to measure analog so I want to first make a voltage of around 8-12Vdc then use a linear regulator like LM7805 to make 5Vdc.
3. I need module only. SO I checked for dc-dc buck. they are very costly >$100.
4. For lost cost option I see some people use a resistor & zener first to drop the voltage but I didnt find this good solution.
5. Ground of 40V-100Vdc should be same as of 5vdc & system should be non-isolated.
Any low cost solution for this. I want module based system in this.

 

These two can be used. I am using LM one.

http://www.digikey.com/product-detail/en/xp-power/RDD08110S12/1470-3083-5-ND/5726803
http://www.digikey.com/product-detail/en/texas-instruments/LM5017EVAL-NOPB/296-37050-ND/3640503

 

Is this 1 off or will there be any volume?
You could roll your own for cheaper than an off the shelf module..
http://www.ti.com/lsds/ti/analog/we...ntr_en&HQS=sva-web-webdesigncntr-vanity-lp-en

 

You could roll your own for cheaper than an off the shelf module..

I doubt that's true in general.

 

4. For lost cost option I see some people use a resistor & zener first to drop the voltage but I didnt find this good solution.

That would dissipate 60W for a 10V @ 700mA output from a 100V supply, so that's not a good solution.

 

I doubt that's true in general.

My comment there was that they could use discrete parts,etc.. and make their own/pcb,etc... if there is any decent volume it would certainly be cheaper than $40..
Now I did not include Design/Testing,etc.. costs into that..