how much does it cost to purchase/register a .com/.net/.org domain?

Normally from 4-8$ a month for a standard domain.

If you want to set up a big website though, it would need a maintained domain server. which may cost from 150-10,000$

this is one of the low priced websites for domain purchase.
https://www.godaddy.com/

If it is for your girl friends birthday i suggest you go here
http://www.webs.com/

it will make a free website that you can treat like a real one but its free.

info domains are $1..99 coms $9.02 net $9.99 org $9.99 link below

http://low-cost-domains.co.uk/

you could use free hosting

http://www.000webhost.com/

Take a look at 1and1.com

They provide free domain name registration for the most popular domain types as a part of their hosting packages, which are also very cheap.

I used to like GoDaddy, but their control panel is an undecipherable nightmare of conflicting information, and when you log on to your account you can hardly tell you have done so. It seems that all they want to do is up-sell you, which I am sure is their main purpose.

1and1 has a control panel which is as clean as can be, and is ez to use.

godaddy.com

9.99 for a year !!

I'd go with companies like Doteasy or Lunarpages. You can get a domain from Doteasy at $25/1-yr and it comes with a free hosting account (no banner ads) with tons of great tools. If you need more hosting features, then go with Lunarpages, US$4.95/month for unlimited storage and traffic and with scripting support, and most of all, comes with a domain name, too. GoDaddy is good for domain names, but I find their hosting a bit lacking and their free hosting comes with their banner ad.

You can find step by step guide on how to put your own website up at website like http://advancescript.com/webdesign.htm ,etc .

it really varies but you should google it just go to google and write down domain names and click the link that you think is best.