Make sure the sandstone is laid rightway up Believe it not it gets laid wrong side down a lot. The other thing to be aware of with ndian sndstone is thickness seems to vary considerably, take a look at it in crate at suppliers. I was going to use sandstone, but decided against it due to lack of consistency in stone. You can tell me it's natural product till the cows' come home.
I think yo need to try qualify garden with dimension's, amount of hard landscaping, additional plants, grass etc.
How long is a piece of string?
Not trying to be smart but we did a garden last summer and they spent 15K on Plants alone !!!
Other gardens fully lanscaped 4.5K
It really depends what you want to spend and then tell the contractor to design around that budget.