The barriers to trees are NOT money. Trees are a question of land use policy: you need permeable, non-compacted surfaces for them to live.

You can either do that by cutting bigger tree pits (which would have to take away road or sidewalk space) or by spending money on more complicated engineering solutions like excavated grow pits, permeable pavement, and/or custom drainage (these things are common in cities like NYC that have their shit together).

I don't expect Philly to start investing in fancy engineered street infrastructure any time soon, BUT it costs ~nothing to turn a parking spot into a tree pit big enough to sustain a big shade tree like an Oak or a London Plane.