Does it fit? If so its a decent bike.

If you go cheap ordering off the internet you might be able to get tires and tubes for $20-$25. Saddle cost is sometimes pointless to calculate because people would change that on any bike to wht they like. At this point its free, but I suppose you could calculate the $20 or so someone might give you for it in its current condition.

For your skipping problem it could be as simple as adjusting the derailers or just a new chain. Like someone else said converting it to a singlespeed might be one way to go.