Numbers are handy to know, but most often prices are written on paper or shown on a cash-register or on a calculator. On your fourth day you are not ready to bargain for antiques on Jalan Surabaya! 

When spoken, prices are usually in thousands and hundreds (for example Rp. 10,500 is ten thousand, five hundred). Understanding numbers when spoken takes some practice. Another perplexity is that when discussing prices, often the units are omitted. If a figurine is quoted to you as "Enam (six)" and you don't know for certain whether they are talking about six thousand or six million, you probably shouldn't be shopping there. 

The basic one-to-nine numbers are handy for spelling out addresses and giving shoe sizes. These are usually spelled out as in 147 (satu-empat-tujuh for one-four-seven). Don't worry about the hundreds and thousands, it's only your fourth day. 

An Australian mate of ours managed to successfully bargain for goods in Bali using only the numbers from one to five. This approach is not recommended.