Is PLN 4,393 the rent payable to you alone, or does the advertised figure bundle utilities or building charges? That distinction could move the net result considerably. I’d also ask how the expected rent was established and model a lower figure between tenants, not just a vacancy period.
I don’t think the cheapest proposal is automatically false economy. If Naomi can coordinate some tasks and only needs listing access plus competent introductions, an itemised service may be economical. The important comparison is the realistic package she will use, not the fullest package available.
There are two plausible approaches here: adjust the recorded sale, or infer a discount from the live listings. I would trust the first more, but only after checking when it sold, how close it is and whether its layout is genuinely similar. The listings mainly tell you what nearby sellers hope to...
Also establish whether €3,833 reflects completed lettings or merely advertisements. Owners can ask anything; the useful figure is what comparable tenants actually agree to pay.