The file attached to this record is the author's final peer reviewed version.Estimating trading costs in the absence of recorded data is a problem that continues to puzzle financial market researchers. We address this challenge by introducing two low frequency bid-ask spread estimators using daily high and low transaction prices. The range of mid-prices is an increasing function of the sampling interval, while the bid-ask spread and the relationship between trading direction and the mid-price are not constrained by it and are therefore independent. Monte Carlo simulations and data analysis from the equity and foreign exchange markets demonstrate that these models (especially SHL2) significantly out-perform the most widely used low-fre...