I'm currently fermenting a 60 gallon olive drum of fish scraps from my local fish market. I'll be using this fertilizer in fertigation but it could be used foliar, dilution 500-1000:1. Every day at 11 am I pick up 1-3 5 gal buckets of heads, guts, spines, etc. and put it in the drum. I began by culturing local bacteria at home in starchy water from rinsing white rice. I further multiplied this culture in 5 gallons of milk which favors the bacteria lactobacillus, a common natural bacteria. That yielded me a cheese/whey mixture loaded with the bacteria needed to digest fish, a "starter" sort of like yeast which I could use to begin the fish fermentation.
When I put in about 4 buckets of fish, I add 25 lbs sugar and a gallon of culture. The drum is well sealed and equipped with an airlock to allow gas to escape but keep air/critters/bugs out. Very similar to wine or beer making but a larger size. Today, after 3 weeks, I finished filling the drum, it will now sit closed for a month or 2 for fermentation to complete. Surprisingly, it doesn't smell like rotten fish, but also not like beer or wine either. I don't think it would be acceptable in a typical backyard but unless you get 20 ft away you probably wouldn't know it. Likely if I didn't have to keep opening daily the smell would be less.
I'm told the end product will be less smelly than typical fish emulsion or worm casting teas. I'll let you know in a month or 2.
So, I expect to end up with about $500 worth of fertilizer. I'll strain out bones using a screen and paint strainer. Cost to me was time and labor $25 for drum, $20 for buckets and airlock, $20 for milk and $100 sugar. I intend to develop a way to get fish scraps during a season when it is more available, and will probably try using green papaya for pre-digestion of fish since it contains high amounts of the enzyme papain to break down proteins.
I have a second drum which I'll start soon with maybe crab/shrimp waste, and when the current fish batch is out and packed into 5 gal buckets for storage I'll be doing the same process with Sargasso seaweed to make my own seaweed fertilizer. I'm on the lookout for other sources of local stock materials I can process which may be high in micronutrients.
As for effectiveness, at present I can't say, but expect the NPK to be comparable or better than commercial products like this:
http://www.7springsfarm.com/browns-fish-fertilizer-2-3-1-55-gallon-drum-in-stock-359-00/