Ah ... this is a very sticky subject ... with a complex answers.
First ... yes, you do have to invest in yourself to develop a portfolio and get a zed/comp card that is strong enough to interest prospective clients. You are self employed ... you need pictures of your product (you), so that you agency can promote you.
But I need to ask you this ....
Who are you pay
ing to get those test shots?
The reason I ask is that you should be paying the photographer directly ...
not your agency. If you are paying the agency then you've hooked up with the wrong people ... they are making money from you for the photography ... not from finding you jobs. Rule of thumb ... never pay the agency directly for your portfolio, your pictures, or your printing. Now, I don't know your agency, so I'm not saying anything is wrong here .... unless they are charging you upfront ... and you are not paying the photographer and the printer instead. Then you need to ask some more questions and do some investigating.
Also ... If you have a really great look , the agency should also have lot's of photographers who want models for their own creative tests and don't charge. I'd ask a lot more questions. Most agencies have a ton of photogs just chomping at the bit to use agency models ... at no charge. Sooner or later, you will need to pay an occasional photographer for images, but it should be one of the best fashion shooters in town ... not just some unknown.
Just a word or warning to anyone who is thinking about becoming a model .... there are a lot of modeling scams out there that purport to be legit agencies ... but they just make their money on wannabe models dreams.
Learn more at these sites about avoiding scams:
http://www.modelingscams.org/upfrontfees.html
http://www.easybackgroundcheck.com/