ALA comes in two isomers L-ALA and R-ALA. The isomers mean that the chemical composition is exactly the same, but the way they are put together is different. One form (R-ALA or right handed ALA) is the one that is better absorbed and thus used, and the normal ALA is not effective. The R-ALA needs extra processing and filtering to extract it ( the manufacture process produces both together, like table sugar is mix of sucrose and fructose, which are isomers of sugar.) This extra processing makes the R-ALA more expensive but more pure. Cheaper brands are generally a mix of R and L isomers, so less effective.
As pointed out above, it may be more effective to pay the extra for R-ALA but reduce the dose.