One thing is for sure, a good flip is hard to find. Detroit is a tough market. Like Gary said some of those homes are in real bad shape.
The very best book that I have read on flipping house is called "Flip" this book will give you step by step instruction. If you fallow the book you can`t go wrong. Below is a Link to Amazon
Buy the Book Jesse
Yes, you can offer less, if the home is worth less than the asking price.
You have gotten good ideas. The first thing you need is a budget, you need to factor in repairs that may need to be done. How will you get these repairs done? Will have cash or will you need to finace this in. Flipping has been a great source of income for many people. Check to see what current sales are like in the area you are interested in. If sales are slow, or inventory is too great, it is not the right time to flip. It would be better to use as a rental income property until the market shifts. Just a thought.
If you rent and use a property manager last year they started at $250 per month plus costs incurred.
If your question is, how much are fixer uppers in Detroit, homes are available starting at $5,000. Of course these may not have doors, plumbing, or siding because of thieves.