Based upon what you have mentioned :

Full Y- class fare on AZ from LHR-ROM is GBP 876 return ( USD 1750 ).
Full Y- class fare ROM-BOM is EUR 3427 ( USD 4650 )

Hence total ID90 fare for LHR-ROM-BOM should be about ( 1750+4650 ) / 10 = USD 640 return.

Note that even if you are going one way it is normally cheaper ( in the long run ) to get return tickets ( if your credit card limit can survive it ) and refund the unused sectors ( half the return fare is normally about 70% of the cost of a one way fare ).

If you do not have ZED agreements with other airlines ( which are mileage based and has far cheaper fares ) perhaps you should consider a low cost commercial ticket which may be about the same cost as your ID90 ... for London to India try www.jetairways.com.