We know from big bang nucleosynthesis that baryonic matter accounts for about 5% of the universe's total mass-energy density. What is the current best estimate of how much of this is in the form of stars? I'm guessing that this would be known only very roughly. It seems like you would just have to survey large volumes of space for stars and larger structures made out of stars. Although galaxies may pretty efficiently cycle their hydrogen and helium through stars, I would assume that there is a lot of hydrogen out there in the spaces between superclusters that has never had a chance to form a star and never will.
"Only about 10 percent of the total baryonic matter is sufficiently condensed by gravity to form stars and galaxies. More than 90 percent was left between the galaxies."
6% of baryonic matter is within stars according to the following lecture: