At this far into a thread it's good to find out what the original poster said...
. I intend to daysail and overnight out of Portsmouth Harbor (NH, USA) with cruising trips once or twice a year up the Maine coast for a week or two at a time.
look at a salient point... Below
You will die from hypothermia if you are in the water long before they even decide to come look for you. Literally.
A few days ago we discussed the sailing ship Concordias loss off Brazil in 2010 the EPIRB took one hour to send the GPS information to the Brazilians (who then didnt respond to it for 24 hours).
ALL EPIRBS take about an hour to choof the info off. Hopefully the US coast guard then does something with it in under a day...
If there is a threat of hypothermia the first hour is crucial. Absolutely, positively crucial.
If the OP is doing as he says, and not lying to us, and he is costal hopping, costal overnighters, why wouldn't he find it much quicker to ring the CG on a cell phone?
A cell phone in a zip lock bag are waterproof.
An instant response by the CG is more likely. On the coast the CG may be able to get to any scene in under that magical hour.
EPIRBs and PLBs are great. I wouldn't go anywhere without one. But if the OP ONLY HAS $700 and is not lying to us, what way is the money best spent?
I can't see an EPIRB or PLB will outweigh an AIS transponder, which as someone pointed out can be used in fog too... An AIS can be used every day in every navigational situation. In emergency when someone rings the CG they already have the position on their computer, or the last position.
If the boat has sunk and the AIS not working, the phone calls, and flares will still get a rescue quicker than that hour wait.