Values for M. Ugarteburu guns are not listed in the Blue Book.
It was one of 22 Spanish gunmakers that combined into one firm known as DIARM in 1985. Most of them, including M.U., did not survive when DIARM went belly-up.
From Wieland, "Spanish Best":
Martin Ugarteburu produced some very fine sidelock doubles. One catalog showed a dozen different models, all sidelocks, all high grades (Model 108 through 118, plus Model 125; the suffix "A" denotes automatic opening), with the Model 117 being an exceptionally showy piece with scroll engraving down the barrels and tang. In 1972 the company is shown as producing some boxlocks as well.
Is yours a boxlock or sidelock?
You can get the year it was made from a code stamped on the barrel flats and/or the action flat. The key to the date codes is here http://www.9mmlargo.com/year_codes.htm