You might want to review this info: https://www.adobe.com/products/photoshop-lightroom/compare-plans.html
If you sync photos from Lightroom Classic to Lightroom's cloud storage, only 2048 pixel "smart preview" image copies get uploaded, and these do not count against your cloud storage used. So 20 GB storage is a lot for Classic. However, if you also want to use Lightroom Mobile, 20 GB can fill up quickly. The 100 GB mobile-only plan is the cheapest plan and comes with more storage. If you also want to use Lightroom on Mac or Windows, the 1 TB plan is a good value -- just remember that this plan does not include Lightroom Classic.
Hopefully one of these options is good for you.
Regarding occassional discounts, I don't recall seeing this happen. Occasionally, Lightroom Mobile will price-test a cheaper 40 GB mobile-only plan, but this isn't available all the time, and is not available in all regions.