From what I've read, a diabetic cat can cause you some pretty steep
up-front costs to get the problem in hand, then the prices decline.
It will still be more than what you spend on a non-diabetic cat.

I had a diabetic cat that I had to put to sleep.  When diagnosed his
sugar was through the roof and several vets (I priced around that day)
quoted me some hefty fees to get it under control.  Combine that with
the fact that I work very strange, rotating-hour shifts and I just
couldn't be there to check his sugar and monitor his
eating/drinking/whatever habits that I would need to monitor.  

After a lengthy discussions with my vet, I decided to put him to
sleep.  He wouldn't live a quality life with me as I couldn't properly
take care of him, and his chances of being adopted were very minute,
as he was 15 y/o, diabetic, and VERY much a one-person cat (he ran
away from everybody but me).  It was very nerve-wracking to put him
down, but it was the right thing to do given the circumstances.  It's
been four months since I lost him, and even though I have a new kitten
it's still just not the same in the apartment.

If you feel you can afford keeping your diabetic kitty, and can
provide Toast a quality life while monitoring him as necessary, then
great!  I hope you can and wish you luck in it.

Yup.  Ive been told Ive been very lucky with Em, however at the moment it
doesn't feel that way as Im having a hard time regulating her sugars.. but..
on with the story.

I found out several years ago actually that she was diabetic, because she
was limping.    I don't remember the details, but I do know the vet kept her
for a couple of days to make sure the insulin levels were what was necessary
to maintain her.  We put her on humulin L.. which is actually a human
insulin.  It is relatively inexpensive, and we were told to give one shot
with breakfast, and one shot with dinner.  Now that I work in a vet clinic,
Ive been having her checked out more frequently, and I found her sugars are
very high, so we increased from four units, to five, to six, to seven, then
switched her over to PZI.. a very expensive insulin.. but one proven to have
longer staying power.. and despite increasing the insulin, her sugars remain
high. (300 to 400 range normal bg's should be just over 100).. so we are
starting to think she's insulin resistant.  I just went up to seven units of
PZI, and will test her sugars over the weekend.. I also have access to an
herbalist, and we are going to try some herbs for her that are known to help
diabetics.  I do her testing at home, with a human glucometer.  With her
insulin needle, I prick a vessel in her ear, and get the blood that way.  I
do it on a Saturday or Sunday when Im home for the day.. it only takes a few
minutes, and they recommend four or five readings through out the day to get
a curve, to see how she's handling her insulin.  I also only do it once a
month, or once every couple.  Once she finally finds a way to keep regular
sugars, I probably wouldn't test her for several months.

Typical days are very easy for her.  she eats, she gets injected while she
eats, she goes and spends her day as she wants.  She doesn't get a whole
heck of a lot of exercise despite having four other cats in the house, but
hey.. she's happy.. that's what matters.

HOWEVER.. diabetes in such a young cat is kinda rare.  Last summer we had a
kitten come into the clinic who was diabetic, and the new owners surrendered
it, and one of the vets adopted it.  Unfortunately his sugars couldn't be
controlled at all, and late last fall he started having seizures and we had
to put him to sleep.  I don't say this to scare you, but so you can be
aware, and maybe talk to your doctor about the fact that he is so young..

taking on the responsibilities of a diabetic animal sounds like a lot, but
on the whole it really is pretty manageable.

I had a dog that had diabetes, it was not hard to take care of her.  I had
to monitor her urine and make sure she ate after her shots.  She did well
for a year and a half.  Eventually a combination of old age and liver
problems started to take it's tool.

Make sure you keep sugar on hand in the chance she goes into shock.  Mine
had one diabetic seizure because her sugar got too low.

I've seen cats and dogs live a long healthy life with diabete.  (I worked
for a vet years ago).  The needle is so small they don't even notice it.
Kudo's to you for not being scared off!!  :)