nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases. although denser than the space surrounding them, most nebulae are far less dense than any vacuum created on earth – a nebular cloud the size of the earth would have a total mass of only a few kilograms. (wikipedia)
Yes, Planetary nebula are small objects, having a radius typically of 1 light-year and containing a mass of gas of about 0.3 solar mass. To find the mass you need estimate the nebula's density, either on the basis of how much light it emits or (more likely) how much it extinguishes light from objects behind it. Then you multiply by the volume, which is easy to estimate assuming that the nebula is even slightly symmetric.
