If you're concerned about not leaving your cels in your car,
may I suggest you check them with the hotel porter and
retrieve them when you're ready?  That way, they're locked
away and you don't have to lug 'em around.

As for cosplay, there's a toughie.  I'm much more of a 
cosplayer than a cel collector and I can tell you straight
up that nope, good cosplay is _not_ cheap.  I've spent
anywhere from $35 to $400  in making my costumes; I average
about $150-$200 per costume.  Then again, I also insist
on items like wigs and accessories that can really drive up
prices--I'm a stickler for detail.  It's a fun hobby; you
won't feel like a goof for being dressed at a con ('cause
there's safety in numbers and there are always plenty of
cosplayers), but it is indeed pricey.  There are ways you
can save money and make less expensive costumes; as a 
beginning cosplayer you'll probably want to start with 
simpler, less costly outfits anyway.  

One other warning: Cosplay is also very time consuming; I had
several costumes I wanted to show at Anime Central but could
not because I simply ran out of time.  (I did co-host
the cosplay panel though.)  You'd be well advised to make
your cosplay plans far in advance, as you need not only
time to construct the outfit but time to gather the
accessories (including hair, shoes, and makeup), get used
to wearing the costume/work out kinks in the design 
(you don't want to get to the con and find out your dress is
a little tight or requires special underwear you don't have
or your headpiece won't fit through the door), and plan for
transport (I have had costumes that required special
shipping).  Things like where do you carry your wallet and 
room keys or how do you eat or go to the restroom in costume
suddenly take on utmost importance.  

You can do both, of course, and I encourage you to take it up
as cosplay's quite a rewarding hobby--it's all a matter of
budget.