Low cost and a very high return and sets the mood for the rest of the home and allows for more of a "well, I can over look that" when it comes to the normal chips and scuffs inside.
Baring MAJOR issues and each client's individual financial ability and goals. I try not to suggest above 15% of value on very targeted projects.
But when done properly, on a well maintained home, a good coat of paint, and perhaps a new water heater or kitchen appliances, which tends to not cost more than $10,000 in upgrades.
Though I am not familiar with Philadelphia, The National Association of Realtors had a report on Cost vs Value which I found HIGHLY informative and backed my findings of front doors tends to make good returns (See attached link)
There is so much that can be accomplished by organizing, cleaning, de-cluttering, painting, planting, repairing, etc. that doesn't require a major cash investment. This is probably the best place for you to begin. Curb appeal, kitchens, and bathrooms should be given special attention because of their importance to common buyers.
Please consider investing some inexpensive time sprucing things up before commiting to any major initiatives.
A house might be in great condition and need only minimal fixing-up. Let's say $3,500 for paint, minor repairs, etc. On a $100,000 house, that would be 3%. On a $300,000 house, that would be 1%.
On the other hand, a house might need moderate fixing-up. Let's say some work in the kitchen and bathrooms. A bit of landscaping. And painting and carpets. So, maybe $20,000. Now, I wouldn't put that much into a $100,000 property (see Gerard's comments). But that'd be 10% on a $200,000 property, but only 2.5% on a $500,000 property.
So: No customary percentage.
What you want to do is fix/correct any potential deal-killers. Don't fall for the rationale that "I don't know if I want to spend all that money on _______ when the buyers might not like what I selected." It your carpeting is ratty, replace it. If you've got 20 year old wallpaper, strip the wallpaper and paint the walls in an off-white or light beige. People are looking for properties in move-in condition. And they tend to greatly overestimate the cost of repairs.
After you've fixed the deal-killers, focus on a couple of items/areas to really make them "pop." So the entire property is fully acceptable--clean, workable, move-in condition--with a couple of areas that will really "wow" the buyers.
A home stager can give you some very good guidance on what to tackle.
Hope that helps.