Our DiL's family and us have more or less melded into one. They all live 200 miles away, but we stay with MiL when we visit.
There is an economic gap between us. My companion grandmother was widowed when her children were around 5. Her DH worked for British Rail, so she has a satisfactory pension income, but not large and she never worked, and has lived in the same house for 60 years. DH had a job that took him all over the world and, coming up for 80 still works, as and when. we have upgraded houses and have been able to help DS when times are tight. But she is the grandma round the corner and sees far more of DGC than we do.
In our case these differences haven't stopped us being good friends. We have holidayed together, and, except during COVID, always stay at her house overnight when we visit DS and family where they live.
I think you define what your problem yourself I feel very inadequate and compare myself frequently. If you feel like that you will find it difficult to be good friends with your DS's in-laws. Why do you feel inadequate? Just because the other side of the family have more money, so what. I would hate it if my lovely DDiL's mother was feeling - and presumable acting - like you. We have a good relationship because we like each other and see each other as equals - which we are - in our love and care for our grandchildren and affection for our own child and their spouse.
Bear also in mind, that the nicest of people, may actually not have much in common and not seek out each others company simply because they do not 'jell'.
The main thing is to chuck the inferiority complex.