You can definitely find a diamond mangalsutra for under 40000. With this budget you have a lot of options. You can get a solitaire or a pretty floral diamond pendant with a thin gold chain and this will cost you something like ₹15,000 to ₹25,000. A diamond mangalsutra with stones and a dual-tone chain is also possible and this will cost you ₹25,000 to ₹35,000. Then there is a designer diamond cluster pendant with beads and this will cost you ₹35,000 to ₹40,000. Diamond mangalsutras from brands, like Kesav Diamond Touch are an option because they offer certified diamond mangalsutras with genuine gold so you get a good quality diamond mangalsutra without paying too much.