A Series 3 platform TiVo would let you record digital OTA for $12.95 per month maxium, but that figure is for the entire TiVo service which includes a license to use the software as well as the guide info, and they've always sold the hardware cheap with the idea of making up the loss on selling the service.
In fact, you can probably pick up a used S3 or S3 HD or HD XL with Product Lifetime Service for $300 or less (check area Craigslists), then another $100 for a 2TB WD20EURS to slip into it and $10 -$15 for Low ESR 105 degree rated capacitors to replace the ones in the power supply pro-actively, and the only monthly cost will be the electricity.
Lurk at tivocommunity.com for a while.
You'll also find discussion of Myth and WMC there as well.
When you're paying for Aereo, you're paying as much as anything to have somebody else worry about providing you with an outdoor antenna.