That’s bananas. I can only imagine it’s a precursor to phasing out the cheaper option altogether.
For me personally, there’s barely $1 a month in value in the expansion pack, and that’s only on the assumption that more games will be added.
Edit: Apparently it’s not quite as bad in the UK. “Only” double the price. Still way more than I’m willing to pay, though. In fact, this is just reminding me I should cancel my existing sub.