Real world values vs. catalog values are accurately described above. I would only add that any stamps below a certain catalog value - a few dollars or so - usually bring an even lower % of catalog value, particularly from a dealer. Most dealers just plain don't want/don't need the cheapest and most common stamps for their inventory and you will be lucky to get pennies for them. Dealers probably won't even consider the value of those stamps when making you an offer. They're mostly interested in mid-range and higher-value stamps, those retailing for several tens to hundreds of dollars and up.
To get a better idea of what you might actually get for them, you might want to tally up just the stamps valued at around $10 and up, since cheaper stamps are worth very little to a dealer, who likely already has more of those stamps than he knows what to do with. You'd get more for the cheaper ones by selling directly to another collector (via
ebay, etc), but even most "serious" collectors don't pay that much attention to lower value stamps because they likely already have them in their collections. If you have $30k in catalog value but they're all from stamps with a cat value of under $5, 10% is probably too high. If a lot of that value is in stamps valued at $25, $50 or $100 and up, 10% becomes more realistic and might even be a bit low depending on what you have. There are a zillion factors in play and, as others have already mentioned, condition is probably the most important, particularly on the older ones.