It's the same. I already know I'll be traveling out of the country 3 times or more every year so I always get the multi. I can't even remember the last time I've done a 90 Day reporting.
Harder than some, easier than others. Compared to a retirement extension, for example. Spouse visa is MUCH harder if you don't count the income requirements.
Use Google Flights and set up an alert to send you an email when prices are down. I have mine set up to fly to the US from Phuket and sometimes get alerts for around 25,000b, around $800, I think.
What a lot of organizations do to work around the WP for Volunteers is that they call a paid experience as volunteering. They still call it volunteering which is ok, I guess, but in reality, the person is paying for whatever experience they are doing. So technically they are not "volunteers." They are paying tourists. This is really common in the elephant tourism industries here.