Since you boil cooking water anyway, extra boiling would be for drinking water during the day. I have coffee with breakfast (boiled) and tea at dinner (boiled) as drinks. I actually only drink 1-2 quarts of water that would need to be boiled beyond what I do with meals. I think the 1 gallon per day is over estimating what needs to be additionally purified.

On a short trip of 2-3 days the extra fuel for boiling would not be a big issue. However, it would be for a 10 day trip.

You could also boil up water and have tea at every rest stop.

I just got a Sawyer squeeze filter at Walmart for $29. It is a smaller capacity (1 liter) but may be within your budget.

You should check with the agency that monitors water quality in your area and find out exactly what is in the water so you can decide what level of purification you really need. If there are really nasty contaminants that do not filter out or mitigated with boiling, then you may have to consider hauling water just like desert backpackers do. In most states, the really nasty water is under pretty detailed monitoring programs. USGS also monitors some streams.