I've spent last days to compare different payments gateways solutions. And I've seen that if I want to integrate a payment service (such as paypal, or authorize.net) into my website I need to pay a monthly fee (and not only transaction costs).
Is this correct ?
For these reasons, I saw it is much more convenient for me to use Paypal Standard Payment Method and to only pay transaction fees.
Could you tell me if I'm missing something, or something is wrong ?
You understand it correctly. The vast majority of payment services have monthly fees associated with them. This includes a merchant account monthly fee as well as a payment gateway monthly fee. If you wish to avoid this fee you can use some of the services offered by third party processors such as Paypal Standard and Google Checkout. The downside to these services is that you lose flexibility as these services typically do not allow for a seamless integration into your website (i.e. your customer has to leave your website to make payment). It's a trade off: no monthly fee vs customization. If you want to avoid monthly fees go with services like Paypal Standard that don't have monthly fees. If you want full control over the checkout process use a service like Authorize.Net.
