Gas giants and magnetic fields are 5 each; a close-range scan of a black hole is... 40, I think? You can also get a good chunk of data by throwing some volatiles at a sensor platform (if you haven't already done that in the area - it'll tell you if you're not going to get survey progression out of it), or pinging the middle of a nebula (as in the nebula-type star system).
The trick to scanning neutron star beams is to approach from behind; you want to hit the sensor ping just before you enter the beam, and then it's rotating away from you so it costs hardly any supplies to get your scan and get out again. (And note that you get credit for the beam and the star itself separately; for the latter, you have to be in the star's corona. It's technically possible to get both off a single ping, but the timing is tricky and unreliable.)
...The trick to getting close scans of black holes is to have s-modded solar shielding on everything in your fleet, though. (Which I do recommend, it's really nice to be able to use hyperspace storms for mobility without them eating all your supplies, but if not, well, maybe put your fleet in storage and go wandering about with a dram or something just to get the scan data...)
Long-range black hole scans are much like the neutron star scans: just dip in and out again. (Worth noting: getting a short range scan on a black hole will not give you credit for the long range scan, so if you are wandering around with s-modded solar shielding, you'll need to scan every black hole twice.)
It's also worth noting that you can get scan data off of things in the core worlds; throw a sensor ping when you head into a system via a gas giant's gravity well.
