Ilfotol is available in 250ml and the equivalent from other manufacturers comes in even smaller pack sizes. You mentioned that it is a "washing aid" - no it is not. The Ilford washing-aid (used to remove fixer by-products from fibre-based paper) is called "Washaid" and is not required for film. Ilfotol is a wetting-agent, to be used in the last rinse of the negs before drying them, and is not required for paper.

If you want to use ID11, then remember to dilute the stock solution immediately before use, not at the time of mixing. If you buy/mix a film developer in a one litre packing (liquid devs are also available in much smaller pack sizes) then split it into smaller bottles with no air and use them in turn. If you wanted a more economical Ilford liquid developer then look at Ilfotec-HC, which can be thought of as the Ilford equivalent of Kodak HC110. If economy is very important then you can use D-76 in place of ID11 and it is half the price, or less. Liquid developers are also available from other manufacturers at half the price of DDX.

Is the reason for using Ilford products (apart from the excellent quality) local availability? It might be cheaper to order from Europe, USA or Japan?