Right.... Needs to be a moderate sized order & if you are sure you will order at least once per month for more than 600 for 3 months then you do get the 300 rupees back. But if you want to utilize other bank offers, slowly all/most offers are now for more than 1500 and many give only 5%.
Another thing. DBS-BigBasket Co-branded debit card gives 5% for non BBstar members, but 10% for BBstar members - And there is no minumum amount, though to avoid delivery fee you need to keep up above 600 even with BBStar. But cashback is after 3 months or something, so no instant gratification - but those who are patient and want to get good VFM - then can consider.
On Grofers, Amazon Pantry & Flipkart Supermart - Grofers has the equivalent SmartBachat. Amazon Pantry regularly has similar offers on using Amazon Pay and Flipkart has the Grocery Pass. Each have their merits and demerits. I usually check these 4 places and plan my orders where I can get the best deal - sometimes I even split across based on item prices. I mean I dont see the point of 10% discount on a 500 rupee item, when its available without discount, but for 400 in another place.
I have both BB & Grofers membership. Grofers is cheaper compared to BB. Regarding cash back u will get the 300 u pay as cashback in 3 orders u make (They will credit 100 for The first order of every month for 3 months) technically the membership is free. But the prices are already inflated to match that cash back.
For non-member free delivery is only above 1200. For member it is above 600. So if you are a regular user and normally order between 600 to 1200, it makes sense. Also, members get "priority" delivery slots. During initial few months of covid, I was getting delivery slots as member while many of my friends were not getting any delivery slot as they were not members.
As for rates, I guess it differs from place to place. I had both BB & Grofers membership, free through Times Prime. But found grofers rates to be mostly higher than bigbasket. So decided not to renew grofers membership. Bigbasket GV are regularly available with 5% to 10% discount and can be added to wallet for future use. That also is an advantage over Grofers.
Prices seem to vary based on product and also sometimes the same product is cheaper in one place as compared to another. That is why I said cannot suggest any one based on price alone. Sometimes, its also the availability of a particular product itself. For example, there are some items which are available only in Flipkart or Amazon Pantry & similarly there are some brands or specific product within a brand is available in only one of the places.
Grofers is more like local kirana shop, limited items at cheap price. Bigbasket is like grocery store in mall, exotic veggies, imported stuff, wide range, little costlier but you can get a good deal if you visit regularly and know how to select.