A precancel on #702 may be more desirable than some, but hardly "rare". I just checked sold
ebay listings and a couple of different precancels on #702 sold for between $1.50 and $2.25.
Someone with a Precancel Catalog may be able to give you more information, as sometimes the rarity factor increases based upon the specific precancel location rather than in the stamp itself.
It's certainly collectible, but I doubt it is all that valuable, especially with a precancel from such a large city such as Philadelphia.