To give you a reference:
My agency charges anywhere from $1500 to $7000 a month for Digital Marketing work (India is cheap!). This may include a combo of: strategy, content creation, execution, social media handling, ORM/CRM, ads (PPC/Display/Native/etc), reporting and analytics. On top of our retainer, we charge a 10% media handling fee for all media spends (which come out of the client’s pocket) and sometimes success fees when we meet contractual goals (eg: $1000/month for a 10% growth in bookings, etc).
Coming to your situation: In a European country, EUR 1200/month for a single person doing just posting and no content creation seems reasonable to me, especially since, as you say yourself, this is your first gig of this sort even though you may have experience in this field. That said, you will know what work you’ll be doing only after you get into it - something we’ve always experienced with our clients. Clients ALWAYS say “not much work” and it ALWAYS turns out to be more.
Whatever contract you sign, make sure to put in a renegotiation clause in 3 months.
This helps to upsell your services to clients, and get out of bad contracts whenever needed. Over-deliver in these three months, try to make clients see your worth, and increase fees. It might also make sense to put in the number of hours you expect to work on this brand per day - say 2 hours or 3 hours. You can find out how much a full time guy is getting at their company per month, and see what fraction of it is Eur 1200 and then decide what number to put in there - another way for you to argue to increase your fees down the road if you want to.
Coming to what additional you can offer: It’s always important to keep offering new things to clients and keep them interested enough to continue your services and set up the case to increase your fees. I would say something new every quarter to keep them on their toes. Various things you can do:
Audience Research - Use Facebook’s Audience Insights tools or various 3rd party tools (social listening) to give the client some interesting data about their audience that might help them create future campaigns.
Split testing on your ads & give them valuable insight on their audiences - Give them data on what kinds of caption are working for them, what age groups they’re targeting vs what age groups are interested in them, the demographics / behaviours of their audiences etc.
New kinds of ads - Carousels, Canvas ads, Stories, Collections, etc. - Try to show them how your ads are increasing engagement. If you have or can get access to their website analytics, try to show audiences going to their website from your ads are more engaged that other audiences. Show them you’ve experimented with various kinds of ads and campaigns to get them the maximum juice out of their media $$. I have an alco-bev client, and one of their primary strategies in Europe is to host/co-host a lot of events there. You can try to improve visits to these events with your ads and show them how you’re doing it.
Content / Campaign Co-Creation - Since the bulk of the content creation is being taken care of by an agency, you really can’t do much there. But the way you put it, I feel their agency is a mainline creative agency, with no hand in Digital. You can ask to be involved in campaign creation from the beginning so you can suggest ideas of taking their mainline campaign digital - interesting digital hooks that will improve their campaigns, creating a landing page to unify campaign efforts, etc.
Beyond Facebook / Social Media - This is where you can actually make some easy money and impress clients as well! If your client doesn’t have a dedicated media buying agency, you can try some non-social ads for them - Adwords (easy), Taboola/Outbrain, etc. You can also try to get them started up on new social media that they’re not using, like Flipboard (would work for, say, a single malt brand) or Tumblr (would work for a beer brand)!
Hope that helped or at least gave you some ideas.
All the best, man!