Hi @BaileyHorse . I have an elderly dog (around 14) who was recently started on Fluoxetine. After the initial course, we went on to Fluoxevet - the same thing really, just obviously Fluoxevet is licensed for animals.
It was a slightly different situation to yours because she was prescribed it as she'd started becoming anxious, confused and restless, very likely due to the start of dementia, but honestly it's been terrific for her!
She'd been getting so worried overnight when she couldn't find us (her eyesight isn't brilliant now either) that she was wandering around, panicking and pooing in the house if we didn't hear her quickly enough, which was so sad. She literally hasn't done that since and she sleeps much better. It certainly hasn't slowed her down in any other way, she's still full of life and fun, she's a brilliant wee dog.
We pay less than £30 for a month's supply from the vet. She has half of a 32mg tablet a day. She weighs about 12.5 kilos, so probably not much different to your dog.
I know all vets vary in price of course, but certainly we only have positive things to say about the tablets.