As Andrew Leach says in his answer, it is not not a real word; in fact, Google Scholar shows 1880 results of the word "estimatable".
While not a large enough number of uses to make it standard usage, a quick scroll shows that most of the publications are in mathematics and statistics (and, of course, here the results are limited to scholarly articles). The sciences often "create" words, and these words can well become standard usage within a particular field. A similar example would be "rounding", as in "going on rounds", commonly used in nursing but not in standard dictionaries with that definition.
My search for "estimatable" in normal Google mostly brings up either the same sort of usage or questions about its usage (such as this).
As such, I would say it is safe to use in the particular context in which it does currently get used, but not formally elsewhere.
Unfortunately, I don't have any good alternatives off the top of my head.