That is exactly what I was told by Ken, the sales guy, so I went down and met their Director and he confirmed that Riverstone has never changed a quoted price or passed on increase in cost to clients. My mistake was I did not take it in writing. I did take it in writing that I will be free to bring in my own trade or supplies if I see it being more cost effective but not the cost hike.
If it is Ken you are dealing with or whoever it is, tell them you spoke to Shai and I have suggested that you should get that in writing as a part of the contract. They use a standard contract which allows them to pass on the cost rise should they wish. That is what almost made me walk away when they surprised us with the 60k hike. Mark visited us with all his comparative costing from when we signed and current supplier invoices and since any new builder would have charged us the cost increase, we decided to continue. What you are being quoted now likely has the cost rise factored in but given the current times, there is no way of knowing whether it will get better or worse so take it in writing from whoever you chose to build with.
Wrt variations, their interior designer Nadia has a very good sense of style and a lot of experience. Her services are included in their price. Designing phase was done before contract signing and was only for the look and feel and room layouts based on our feedback and needs. The storage/cupboards/WIR/kitchen/lighting all came later and were suggested by Nadia and revised by us. They have been very accommodating with minor variations and Nadia has told us the areas that we are better of getting done ourselves to save the builder's margins. They do not charge like project builders for every small thing and in some cases variations have reduced the contract price for us.
One thing to note is that they allocate 8k for a WIR and that is way too low. Even before price rises 8k would not get you much in terms of a custom hand made designs compliant WIR, now it is worse. This is my first build and I had no idea of costs of things like WIRs , staircases, Lifts, Pools, Landscaping etc. Knowing that helps to check the allocations made by the builder. Lower allocations makes the contract price look more attractive. And conversely they also tend to use the best and most expensive in everything so there is a scope to save money with things like smart wiring/security, Lift, Lighting etc if you know what you want.
Overall everything has been excellent so far barring the 60k hike. Hope that helps and feel free to ping me if you need more info.