If PowerBI cost is worrying you, then it's likely not what you're looking for. I think right now, it's about $9.99 per user where competitors like Tableau used to be around $1,000 per user, but now have monthly subs as low as $35 per user.
PowerBI Enterprise Calculator
In my opinion, PowerBI is the way to go if you can afford the $9.99 per user per month. You have the 10GB per user and per workgroup (groups of users) that is pretty much like SSAS Tabular in the cloud without having to setup SSAS specifically. PowerBI allows you to remodel the data and comes with very nice visuals, filters and much more compared to SSRS. It also allows you to live query on on-prem or Azure-based SQL Servers in case 10GB is too small for your organization.
I would say it's cheaper to stick with SSRS if you already have it, but you would be missing out on some great visualization features with PowerBI if you do so. For $9.99 per user per month for pro and now with the Azure SSAS available for additional BI power, you really can't go wrong.
I think the only thing I miss about SSRS is the parameters you could set on queries that tied to filters. This is non-existant in PowerBI thus far. Other than that, PowerBI is been a great product and certainly one you should try out if you're looking for a good SSRS replacement.