I don't know your level of tolerance for stuff which is not new and shiny, or for learning software, or for lots of other things which come into play.  I'll just toss out what i use and you can consider it on your own.  What I use is a fairly old, cheap, ruggedized laptop with Farmworks Sitemate software and a GPS....I have a $1000 centimetric DGPS because I also use it to run the lightbar.   For just the mapping, a $100 Garmin 16A will do the job almost as  well 95% of the time.  Computer cost $25 when I bought it, Sitemate was around $400 if I remember right.   Been several years ago.   There are freeware programs which might be capable of doing the same thing, but I decided at that point in what we were doing I needed something with someone to support it at least a little, because i was already taking on a whole lot of do-it-myself stuff along the way.     I set Sitemate to record a "path" and then set the line width to the same as my sprayer coverage, and then as I run through the field it essentially paints the field black on the screen as I go.    This helps tremendously when working a farm with lots of little irregular fields.   Without the computer, I basically would treat each field as a seperate job, starting on one side and finishing it before I moved to the next.  With the computer, I can load the rig, spray across several patches to the farthest one, work on it unit finished or nearly empty, then spray my way back to the truck to reload.  It doesn't sound like much but being able to spray out and back on each load instead of just deadheading can add up to quite a bit of production in a day on a nasty farm. 

Farmworks now has Guidemate, which is a lightbar program which integrates with Sitemate.  With it you can do the mapping as I do while also having a lightbar on the screen in the same package.  I've not used Guidemate but from looking at demos it doesn't look like it would work quite as well as having the lightbar seperate from the computer.   

There is commercial equipment on the market from Outback, Trimble, Raven, AgLeader, and others which accomplishs the same thing at much higher cost, possibly with less difficulty to the operator because of not using a Windows operating system.