I decided to make a quick spreadsheet to compare the costs of some of the webcomic apps! Since the prices of some episodes vary within certain platforms, you can make a copy here if you'd like to change those. I believe I've seen some similar spreadsheets before, but I wanted to factor in sale prices as well as the check-in points for Tappytoon's subscription. I personally tend to only spend during sales so I was curious about the price differences.
Note: The "episodes bought" columns are for how many episodes you could buy with each bundle, but I couldn't think of a better way to word it concisely. For platforms that have different episode prices depending on the series, I gave the cells dropdown menus to select from a list. Manta's is a mess mostly because the manga and novels have a wider price range, so sorry about that.
Lezhin is of course the most expensive, which makes sense because it has a lot more titles. It was also a lot more of a pain to understand how the monthly subscription worked, so I gave up and didn't include it.
Something else to consider is that Tappytoon and Lezhin both have daily check-ins while Tapas has a lot of events for free ink (which makes Tapas a lot more affordable than the math shows since I didn't factor that in). I decided not to include the first-time packages from Manta because you can only purchase those once, but I would say the deal you get for your first premium subscription month ($5.99 USD) is definitely worth it. I'm not sure about the regular non-premium subscription because I couldn't find how exactly the "extra gems with gem bundles" worked.
Also lmk if you notice any mistakes or if I missed anything, I made this at like 3am.