DAICEC is a world class convention center that is built with $680 mil budget. I work in a building that directly overlooks the center and it is occupied more often than I thought it would. So there will be competition to BEC however in terms of costs I would imagine DAIECE will be much more costly than BEC. It is suitable for diamond shows or celebrity weddings rather than industrial trade shows. As a test, whenever I park there, I end of paying 250 per day compared to MMRDA lot right next to that which charges me Rs 70.
Reliance is not in the exhibition center business. This center is a drop in the bucket for them. It is also called Jio Garden and open to public when there is no event. So it looks like a branding cum CSR initiative (similar to Dhirubhai Ambani International School which is just a block away). I don't think they will try to compete with BEC in terms of price. In fact, I would say they only want to hold premier exhibitions there to set a standard.
MMRDA grounds just a few blocks away in BKC can be a competition to BEC more than DAICEC but it just an open ground and not a built up center.
Moreover, with Nesco building IT park on its land, BEC will be a smaller share of the total revenue in few years.