Agility equipment

You can buy stuff pretty inexpensively on ebay.
 
How old and how tall is Loki?  This will determine how high he should be jumping. 

I looked on ebay, but it didn't look "inexpensive" to me. I just wanted to see if those prices were typical and if I should expect to pay that much for the equipment. I don't know how much of a commitment I want to make to this yet. []
 
Loki is 18 months old and 23 inches at the shoulder. I think the trainer said he'd jump 26 inches, but that it wasn't good for them to jump that high that often. If I have him practice at 20 inches, is that going to hurt him? How long should he practice at a time?

At 18 months, practicing here and there on 26 isn't a bad idea.  As he gets older you can jump more consistently at 26.  In USDAA, he's old enough to compete at his jump height.  If you want to compete, something you want to watch out for is their jumping style.  If they consistently jump lower than their jump height, they may knock bars more often when they do jump the regular height.  However, varying the height is good because it will help them pay attention to the height and jump accordingly.  I've even read, in Clean Run magazine, that jumping higher than the competition jump height, is good occasionally to really get them to pay attention and extend.    So there are lots of theories out there and only you can decide what is good for your dog at this time.  As a youngin' I'd still occasionally lower them and work on speed or entrances from jumps to other obstacles.  However if you're working on jump form, jump at competition height.  []

As for tunnels, the cheapest one I found was from affordable agility.com.  About $200 for a 15 footer with 6 inch spacing.  If you go this route, wanna help out my agility club?  [linkhttp://www.flyingpawsagility.com.%C2%A0]www.flyingpawsagility.com. [/link] I linked up to them and any orders through our site gives us a little money for the club.  []   The 10 footers are cheaper and I bought the competition variety because we use it in the club all the time.  I do not leave it outside though.  It goes in a storage bin to stay out of the rain and sun when not in use.  I also highly recommend a leash tamer for it if you get one and move it a lot.  Really helps to hold those heavy suckers together!!

Edited: I just checked USDAA and 21" or higher, in the Performance program, jumps 22".  In Championship, he would jump 26".   www.usdaa.com

I agree with tashakota, a good tunnel would cost somewhere around $150-$200 depending on size.  I got my tunnel through my kennel club.  A group of us bought a bunch so we got a good deal, I think I paid about $110 for mine many years ago and it is in excellent shape.  I do not leave mine out in the elements either.  You could get a child's play tunnel pretty cheap but I have heard they do not hold up very well and with a bigger dog tunnels usually get roughed up pretty good.
 
I made my own teeter.  My friend had the plans for one and she gave me the plans, didn't take long.  My teeter is pretty stable, it is a wooden base.  At the time I had a large dog so I wanted to make sure any equipment I had was heavy and could withstand the "vigors" of a bigger dog. []
 
I did recently buy a contact trainer from this guy [linkhttp://www.carlson-agility.com/index.php]http://www.carlson-agility.com/index.php[/link]
 
He seems to have decent prices and my contact trainer is very good quality.  I will say that buying a contact trainer is money well spent.  It has been great, only sorry I didn't have one with my first dog. [] 
 
I would say that jumping Loki at USDAA height would not be terrible.  Personally, I vary it a bit.  If I am working on difficult handling moves, I don't care about jump height as its the handling I am focusing on so I will keep the jumps low or even just leave the poles on the ground between the standards. 
 

A 23" dog has to jump 24" in AKC.  You can jump (starting today, I believe) 26" if you'd rather.  If you're doing perferred, you jump 20". 
 
If you want a teeter, look for plans or ask around locally.  Shipping is a lot for those big items. 
 
Or you can do what I did and buy DH a welder.  []

Loki will jump 24" in AKC.  26" is an optional class.  I think it'll fail, because unless you have a giant breed, there is NO reason for your dog to jump 26.
 
Strauss is 26.5" tall.  He'll jump 24".  In class, he jumps 16-20".  Every now and then we do full height, just so the dogs learn to adjust, but we don't do it often as it can be bad for the joints no matter what the age.
 
As of today, the following things have changed:
 
Only the tall dogwalk is to be used.  No more short one for the little dogs.
 
The up contact on the dogwalk is extended by 3 inches, to accomodate the longer stride of the big dogs
 
Course time is shortened by 5 seconds for dogs in the 12" class (I believe it's twelve inches)
 
There is an optional 26 inch class
 
Dogs may not compete until 16 months of age
 
And there are a couple other things, but I can't remember them off the top of my head.  If any of my numbers are incorrect, please correct me.
 
Also, I do NOT advise working the teeter on your own until your dog has a solid foundation on it with help.  You could easily ruin your dog.