people whom lisa wrote about, who have tried it from home includes me; and i am a programmer for almost 5 years now doing freelance programming.....so trust me hosting from home is a BAD idea...very BAD idea...unless ofcourse you are larry page and sergy brin and planning to start a google from your garage!!!!
but on a serious note, the server software that you are planning to buy will anyway cost you much more than what ur going to pay for the monthly or even annually for a web hosting plan.
And apart from that you need to have technical expertise, optimization if yours is a popular site and you get loads and loads of trafffic, database optimization as well and ofcourse backups of db and files and many more things.............................
so it gets way too expensive than a web hosting plan. so, my advice, buy a web hosting plan!