Remember, TDS readings are truly an inexact science. There is low probability that the reading will be accurate since it is a rough correlation with conductivity that is used to predict TDS and conductivity varies with the types of ions present and their concentrations.
Therefore, its not truly important that you obtain accurate readings. However, its important that the instrument provides reliably, repeatable measurement. I don't think that you have to spend a lot of money on a TDS meter to have a reliably repeatable instrument. I have HM Digital units, but there are plenty of others.