I just upgraded an old freewheel bike to STI, and it cost me about what it would cost me for a new entry level bike.
However, in my case, I started with a like new lugged Columbus steel frame, so for my particular requirements, it works out quite well. (The mechanic didn't think the frame had even been built up before, but I knew otherwise).
What I needed (I went to 10 speed compact):
Custom built 27" wheels on modern hubs (still much cheaper than what many people around here spend for wheels).
STI levers.
Bottom bracket.
Crank and chainrings.
Front derailleur.
Rear derailleur.
Chain.
Cassette.
Rewrap bars.
If you can use 700c (the brakes might reach) you can get off the shelf wheels.
If you go 7 or 8 speed, you can probably skip the BB/crank/chainrings.
Unless your bike is a nice chromoly frame, I would save the money for a new bike in the future.
If you really don't like friction shifting (I don't), then you may want to look at a 6 speed freewheel, a new RD capable of indexing (they are avalable cheaply) and upgrading to indexed downtube or stem shifting as another option to get you some of the advantages without the extra cost of a complete upgrade.