The line does not run between easy and hard work, nor between cheap and expensive. It runs between work that needs brand knowledge and work that needs craft.
What belongs inside
Anything that requires knowing your own company. Who the customer calls when something goes wrong, which promises you may and may not make, why that one product line is sensitive for the sales team. You do not learn that from a briefing, and an agency that finally gets it after two years loses that knowledge the moment its account manager leaves.
Anything that returns every week and changes little. You pay the learning curve once, and after that in house is faster and cheaper.
The judgement. What gets made and what does not, what it may cost, and when something is good enough. You can take advice on that, but you cannot outsource it: an agency that approves its own work is not an agency, it is a department without budget responsibility.
What belongs outside
Work that peaks. A campaign, a launch, a trade fair. Hiring for a peak means having too much of it eleven months a year.
Crafts you cannot keep busy. Moving image, media strategy, technical SEO, copywriting at a real level. Half a day a week is not enough to keep anyone sharp.
Work that benefits from someone who does not know you. Positioning, naming, the question of whether your story still holds outside your own walls. Here distance is not a drawback, it is the assignment.
The sentence that settles it
"If this person leaves tomorrow, what do I lose?" If you lose knowledge about your company, it belongs inside. If you lose a pair of hands or a skill available on the market, it can go outside.
And then, control
Whatever you outsource, someone on your side has to be able to judge what comes back. Outsourcing without a counterpart produces work nobody looks at critically any more, and that costs more than the saving you were after.
That split between inside, outside and which party for what is what IKAg calls the agency ecosystem. See Agency ecosystem.
See also: do I need an agency and how many marketing people you need.