Upgrading an iMac/eMac IDE disk
Upgrade to an SSD
Just get a simple SSD and an IDE to SATA adapter.  You may need a 3.5&quot; drive adapter if you use a 2.5&quot; drive, but those are abundant and easily sourced.

StarTech IDE to SATA adapter (Approx $18 USD)
Kingston SSD A400 120GB (Approx $50 USD)

This will give you a solid state solution for less than $70 USD.  You can probably find these items for less on the used market, but probably not by much when you factor shipping and handling.
That said, your $30 budget is unrealistic as PATA SSD's are notoriously expensive and any small capacity SSDs (16 to 60GB) will run about $20 USD (plus shipping).  In addition, you will still need the IDE to SATA adapter because those cheap drives are all SATA.  You may spend (slightly) less, but it will be a solution based on used (pre-owned) components.
Convert IDE to SATA
The StarTech IDE to SATA adapter will allow you to use newer SATA disks including SSDs.  Whether you choose to go with an SSD or stick with spinning media, this little adapter will be critical in allowing you to use modern drives with your older hardware.

Personally, I have used this exact adapter in an X-Serve RAID (connected to a XServe G4)  to use 1TB 2.5&quot; HDDs (the limitation was 750GB PATA drives).  The X-Serve RAID ran cooler and I increased it's storage capacity limitation by 25% (it was also substantially lighter).  There are other manufacturers of this type of adapter, but this brand seemed to be the most reliable and compact.