How much usage do you find you get on these kinda models (I know the pricing changes a bit) compared to a $20 sub say for Google AI Pro in anti gravity?

I hate how difficult it is to compare prices when looking at subscriptions.

Would $20 in open router, using models like GLM get me more or less?

I think it'd get you less than a $20 sub to any of the big three. I've used it on OpenRouter and found it kind of expensive for the results, but that might change now that it's open weight and other providers can host it/compete with Z.ai. For the work I did with it, I would've rather used DeepSeek V4 Flash just because it's more economical and still gives good results IMO.

Z.ai does have their own subscription, but I haven't used it because their privacy policy was pretty buns last time I checked.

I made a little project to calculate that. It gets the prices for API access and subscriptions, and calculates an average cost per token, for a given rate limit. So the cost is cheaper if you get a higher rate limit (cuz you get more tokens per dollar): https://codeberg.org/mutablecc/calculate-ai-cost

tl;dr API cost (openrouter) is always more expensive than a subscription (for the same given model). you should always use a subscription first and only go to API pricing if you run out of your subscription.

In terms of which subscription is best, different ones provide different models, different amounts of tokens, different rate limits. So it depends on what model you want and how much you need to use it. The frontier ones are always more expensive than open weight ones, but a few subscriptions are starting to include frontier models like GPT-5.6 Luna (which is a great deal but not necessarily the best price-per-performance).