The price for a seizure alert dog can range from $15,000 – $30,000. You can get them pre trained from sites like Paws With A Cause . There is a more affordable option. My Mother has had Epilepsy since she was 18 and has always had dogs. The majority of her dogs were able to sense that my mom was going to have a seizure. They just differed in their response. The most common ones being : Following, restrict movment, and barking at another member of the house hold.
Learn more at Epilepsy Foundation