I've got 0.5BTC to spend. Are there any miners worth buying at this price? Thanks.
No, you don't have enough btc to buy any reasonable miner.
You can add more btc and buy Antminer S3 from the manufacturer, Bitmaintech.
https://bitmaintech.com/productDetail.htm?pid=00020140630025130637RV8OhOwt06BCAdditionally customs duty applicable when its delivered at you location.
The S3 comes without a power supply unit and so you need to buy a one PSU, ATX or Server PSU.
The S3 consumes 340Watts as per specs but need to add a safely margin of 20% to 25% so it becomes 425.
Also keep an eye on the efficiency of the power supply.
The other option you have is to get some shares in hosted miners.
https://bitcointalk.org/index.php?topic=708199Update:
Going for S1 may not ROI if you are buying a new good Power Supply Unit and your S1 is not really cheap.
S1 sales already over and can't buy new S1 now.
I will not recommend buying an old S1 selling as new/old/used.
Always try to buy from the manufacturer directly.
Research well before investing on a miner. You may try good calculators like
https://bitcoinwisdom.com/bitcoin/calculator .
There are bad calculators too so beware of them.