I landed this interview lately with a really good company. Although it is clear from my profile I am not located in the same country as the work, they ask for an on-site interview. The costs to do such trip are rather high, taking into account I might not get the job at all. How can I ask them/convince them for a first online one? Thank you very much
Reply and clarify that you can't incur the costs of travelling for the interview:
Thank you very much for giving me this opportunity. Unfortunately, I am not in a financial position to take advantage of it at the moment. Would a Skype interview suffice instead? Thanks, Andia.
They may offer to cover your costs, agree to a Skype call, or simply cancel your interview. Such is life.
I have to sum up here that communications with the potential new company is very important. I would simply ask them right out if they will provide travel expenses for the interview trip since that will entail travel from where you live to the interview location.
Most reputable companies that are doing external hiring and find a candidate that they feel would be a strong contender for an open position will pay for travel expenses. They may have already made up their mind that they want to get right to the face-to-face interview and so trying to mention a Skype type interview could derail any momentum on their side to see you.