How much cheaper is it to mount oceanographic sensors on elephant seals and other animals, compared using Argo floats and other abiotic sensors?
Each float costs around US$15,000 and communication, deployment and calibrations costs approximately double the through-life cost of each float.

(from http://www.argo.ucsd.edu/Argoflyer_final.pdf)
One can roughly estimate from this source, given a life-span of four years, an Argo float costs slightly over \$3.5k/yr (float only) or \$7k/yr (including deployment and calibration etc.).

...by June 2007, over 95 percent of the goal of 3,000 floats had been deployed...
...With a four-year life span per float ...
...The annual cost of the worldwide Argo program is about $20 million...

(from http://celebrating200years.noaa.gov/magazine/argo/welcome.html#float)
Based on this source all inclusive number will be in the ball park of slightly less than \$7k/yr.
So, without getting into details of whether one can be an alternative to another technically, a primitive cost comparison based on the yearly cost derived from the mentioned article and the links below it is ~7k\$/yr (including deployment and calibration etc.) or ~3.5k/yr (only the float) vs versus \$4k per tag with Tarpon fish. So if one can assume the deployment/retrieval/calibration costs are similar they will cost about the same by the end of first year. After one year the annual cost of a fish tag will keep getting cheaper and cheaper than a float until the end of its life-span.