Soon I will reach the free daily worker limit. Is there a contract for the paid worker plan? And is there an option to set a monthly limit for the budget?
It is not possible to limit your budget. You will start with a Bundle plan, but you can move on a script basis to Unbound, and analyse if it saves you money.
How does that script basis to Unbound works?
After you move to the Bundled paid plan you can select as the image below. The billing when Unbound is selected is more complex, but in several use cases it can be cheaper than Bundled.
Have a look at the billing example [billing example].(https://developers.cloudflare.com/workers/platform/pricing#worker-billing-examples)
This sounds like you run a single script as Unbound while the rest are Bundled. Or do all Workers end up in the same plan?
That option of only a single script in the Unbound plan works.