GoechaLa is perhaps the best trek in Sikkim and one of the best Himalayan treks in India. It's a pretty long trek and graded as moderate to difficult. But the trek is completely worth it. I can help out with GoechaLa if you want. You can also check out my photos here from GoechaLa and my YouTube video here https://youtu.be/E8PXpcV5ihg
For low cost, you can go for the Barsey-Phokteydara Trek or just Hilley-Barsey hike and a lot more. There are quite a few less explored routes in west Sikkim and they are amazing! If you want you can contact me for details. :)