Ok, it came with a 20 so 30 doesn't trip over the next drive size barrier.
As to the price, that's for you to decide. 20 is not a big gamble.
i have a compact evo d510 from 2001. i need an internal hard drive for it and am looking on crigslist for one but dont want to get scerwed. a guy is offering me a 30GB Western Digital Caviar HD,3.5in IDE Type. will that work in my computer and is 20 bucks a good price?
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that's comparable to the disk that's in it now. It's rather typical for around 2001. Open the case and have a look.
In that time a motherboard had IDE-connections. Each supported a master and a slave. So that's 4 IDE-devices in total. The typical PC came with one hard disk and one CD-ROM (or even a burner!). In that case you can add 2 other devices. Say a second hard disk and DVD-drive. Note the master-slave positions and settings (unless it's already cable-select, see the manual).
Are you sure 30 GB is enough? The PC probably would support 60 GB or 100 GB also. That would be a little bit more expensive.
The price is not unreasonable, although (given the law of demand and supply) it should be lower, because a 30 GB disk seen from today is ridiculously small so practically nobody wants one).