For our website, we take the total cost we spend on our website in the year (let's say $100,000 just as an example) and divide that between all the website programs. The cost may come from the price of our CMS software/hosting/etc - not time/labor in-house, but we do include any support fees for out-house CMS support in that. So, let's say you have 40 website programs, we take $100,000 / 40 = $2,500/website program. When entering the period costs for each program, we take that $2500 and divide it by 12 months, so it would be entered as Jan $208 (rounding), Feb $208, March $208, etc...
For webinars that are hosted on our website, they are part of the website programs I just described. For webinars that are hosted elsewhere or that you run through media vendors for promotion and moderation and such, we have those webinar programs separate. We did this where I worked before. So, let's say it cost $10,000 for a media vendor to host a webinar for us and includes promotions, plus a $400 fee to get a copy of the webinar recording. We'd then enter the period costs into that webinar program as $10,000 media host and promotion (with the corresponding month it occured in), plus a second period cost $400 with a description webinar recording fee. This way, you can see the full breakdown of fees. Some people may just sum it all up into one though. Either way works.
In terms of your form and the way that is working now, I can't be sure. You may want to consult with Marketo's professional services to see. Hopefully this helped some.
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