Prince Charlie                        Argyle                                  Jacobite
Rental:   $125-$175                $110-                          $105-
Purchase: $980-$1,380
Source: Kilt Rental- Prom Kilt or Wedding Kilt Rental, minskytux.com

Note: Kilts in Scotland are just as expensive or more than those purchased in the U.S.
      and you have to pay customs duty to bring them home.
      I also looked at buying the material in Scotland and hiring a taylor here.
      That was also more expensive that buying one in the U.S.
      
Note: You can get a basic Polyester blend kilt, sporran, belt and hose for $275.
You can wear a sweater or your own blazer, shirt, tie and wing-tip oxford shoes.
These are considered informal dress, but to my untrained eye they looked just as good.
And you can machine was them.

Highland Regalia
Kilt: Wool (Note yards as a function of waist varies quite a bit. 
      It may depend on the type of pleating.)
   7 yard 	$325-$450 up to a 30" waist *
   8 yard 	$340-$475 32-48" waist
   9 yard 	$395-$525 50+" waist
  Polyester/wool $135 or Poly-Viscose $100
Sporran (purse)†:  	$75-$345
 Sporran chain:   	$25 (sometimes included with Sporran)
Prince Charlie Jacket and Vest:   	$395
Tuxedo shirt & Black Silk Bow Tie: 	 $45
Argyle Jacket:                     	$295
Sgian Dhu or Sgian Dubh (knife):               	 $12-$85-
Belts and buckles:                 	 $40-$60
Hose (long socks) and flashes:     	 $20-$30
Ghillie Brogues (shoes):           	$100

* Waist measurement is taken at the navel to 1" above the navel.
You only need 4 yards of 60-inch wide material for an 8-yard kilt because it is 
cut into two 30" pieces.
The term "the Whole Nine Yards" has been attributed to the material for
a kilt, but it is just one story. See WorldWideWords.org
  

Tartans:
At Scottish Clans and Tartans they say there are over there are around 2800 tartans listed in the Scottish Heritage World Register - including ones for football clubs, individual towns, etc.
Within a clan there can be several types of tartans:

Modern
Ancient
Dress Modern
Hunting Modern
Hunting Ancient

For the inexpensive Poly-Wool / Poly-Viscose kilts many shops only cary three:
The National Tartan of Ireland, Royal Stewart and Black Watch.

See also: Tartans at our Ca Families genealogy site.

What to wear beneath your kilt:
According to tradition it is nothing.

Cecil Adams of The StraightDope.com, posted the question on soc.culture.celtic the old netNews site and got numerous replies such as the following: "Me wears the scotty-skirt and I can assure you that correctly there is nowt unner it, however we 20th century derivatives feel the caul a bit and so unless it's a formal occasion, I actually (NO! No! I can't admit anything ... not in writing anyway!)"

A comment on his message board says,
  "Kilts originally were worn with nothing underneath them because underwear hadn't been invented yet... Indeed, as mentioned in the post, the closest thing to underwear was the fact that the scotsman's shirt was a very long shirt that covered the area in question.
  However, today's modern kilt wearers, people who wear them every day or at least on a regular basis (of which I am proud to be one), wear a variety of things under their kilt. Some do keep it breezy, but many wear different types of under garments for a variety of reasons - an active lifestyle, being around children and/or animals, work regulations, and even comfort. Some wear undergarments on certain occasions and not on others. Some always wear something, and some never wear anything.

Another post at the above board and several other sites say,
"The rule against wearing anything under the kilt was once so strict that up until World War II inspections of Black Watch troops included having them step on a mirror."

There was a web post a while back of a 2004 picture of Queen Elizabeth sitting with the 1st Battalion of the Argyll and Sutherland Highlanders where one colonel's kilt was a little too revealing. Scopes says the authenticity of this is undetermined.

The drunk Scotsman with lyrics - Irish rovers

The real question is, why do Scotsmen wear kilts at all?
Answer: Sheep can hear zippers.

Fort George Journal; Wherein Is Revealed the Answer to an Old Inquiry About Kilts, NY Times 1998
Kilting in the wind/

Amazon.com: They Wear What Under Their Kilts: Katie Maxwell - A humorous story about two girls temporary job on a sheep farm.

Links:
Great Kilt, Poly/Viscose Great Kilt, Synthetic Blend Great Kilt Making a Kilt @ ScottishDance.net
Kilt Measurement
your-kilt.com
The Pipers Cove, Kearny, NJ
J. Higgins, Ltd. -- Custom Kilts
Casual Kilt Buyer's Guide (inexpensive)
Scottish Culture
Tartans
X Marks the Scot - A Community of Kiltwearers..