I assume you intend to perform installation yourself because any shop will charge you around $200 for installation alone. A $300 budget is not enough for a reasonable yet sturdy suspension drop unless you luck out and find a used set on Craigslist, eBay or in a junkyard. Unless you dont intend on keeping your Civic more than 2-3 years, I strongly suggest you save another $300 and get solid quality. Otherwise, you will lose money in the long run.
If you go to a
$500 budget, I suggest Tein S Tech springs and KYB shocks.
With a
$600 budget, get the Tein S Tech springs and Koni Str.T Struts/Shocks. These will last you longer and ride considerably better than the KYBs. Its definitely worth the extra $100.
Here's a link you may deem helpful...
Suspension Set-Ups