What level are you? I found that as I leveled up, I collected coins faster than before. It may require some patience, but if you have a lot of tables and chairs available to your customers and make more costly food that generates more coins per plate, you'll get there sooner. I just expanded for four million coins, and now I'm currently saving up for an expansion that costs six million coins. Patience
Also, you can always expand for gems if you can't wait that long, but I personally wouldn't use 40 (or however much) gems for that.