Well the best thing would be to measure the fuel pressure first. However, on a cost/benefit basis, if you do not have a fuel pressure gauge or are not going to a mechanic first, this is how I see it:
1. Try a fuel injector cleaner like Techron ($7 - $8). BG 44 is good stuff; but, way more expensive than Techron. Going to a shop for a fuel injection cleaning will probably run you from $50 - $100. If your injectors were clogged that bad to dropped your fuel mileage so much, a fuel injection cleaning may not do much.
2. The fuel pressure regulator is like $30 - $40 and is relatively easy to replace yourself; so, not a big hit if it turns out not to be the problem. Gas prices being so high these days, if you don't fix the problem quickly, the cost of the excess gas use will be more than the eventual repair.
3. In Honda's newsletter, they refer to it being possible clogged injector screens, and recommend replacing the injector if debris found. Well the injectors are $70 - $100 each, not including labor. If it turns out to be the injectors, that would be the highest cost.