That sounds like an astonishingly ineffective developing tank. Do you really mean a litre for a couple of rolls of 35mm?! If that wasn't a typo then the first step towards economy would be to replace the tank with a more normal one.
Apart from APX100 and Rodinal (which go so very well together) I seem to use ID11 1+1 for most things, with 1+3 for sheets in trays. This works out at a very reasonable point for cost/longevity when I buy the five litre packs, and is also a very standard and repeatable choice. Occasionally for HP5+ at a higher speed I would use DDX, but there isn't that much difference unless you are going over ISO800.