How does the cost of an ETF change

[brute]

So, looking in Mint today, I see that the cost of a vanguard ETF i have went down. What does that mean? I'm sure its obvious, but I haven't been able to find a satisfactory answer.

[MustacheAndaHalf]

ETFs have some fixed costs.  As the assets of the ETF ("AUM") grow, the fixed costs become a smaller percentage of the fund's assets.  There's also some other economies of scale that allow an ETF to lower expense ratios as they grow larger.