What do you mean "for sale"? Slippage on etf's depends on volume of the particular issue, public participation, volatility, sponsorship, etc, etc. It also changes with time as more people pile on.
For example, 5 years ago when I was first trading IWM and its S&P sister, IJR, the spreads were virtually fixed. 20cent spread on one and 30-50 cent spread on the other. Hell, they had so little volume back then you couldn't even chart the damn things! I was trading them by charting their bid/ask prices.
I traded quite a bit of those suckers and made like $60K in 2-3 mos. Today, the spreads on IWM are very small, but, so what, the volatility and "persistence/trendiness" are gone rendering them very difficult to make a profit.
Also, when I look at sllipage, I think of 2 versions, slippage from bid/ask and slippage from last sale.