Free/low cost with a low learning curve? No. I don't know of any nowadays. But free/low cost with
some or a large learning curve? That's possible.
You might use Mixamo.com for getting started. You have to set up an Adobe account, but the service is free. Then you can, well, try to convert the animations to Second Life through Blender addons
Bento Buddy or
Onigiri. Beneficially, there are many youtube video tutorials on how to accomplish this.