Here is a friendly PSA...
Don't get butt hurt if you ask someone for trade work and they decline the offer. If you really want to work with them and they turn your down you still have options:
Option 1: Pay them. Reply back with a rate you can afford, even if its below what they ask. Sometimes they may be willing to shoot at a rate below their normal rate if the shoot can work for them. My caution is to be wary of the Merc Model, the ones that are strictly doing it for the cash. Even if the Model doesn't care for your work or you are just starting out they should give a damn about helping you develop your skills at becoming a better shooter. Some are good shooters themselves so it's like getting a Model and a photography seminar at the same time!
Option 2: Get better and try again in the future. Sometimes its a matter of getting better at your craft. Your current work may not be at the level they need to get images they can use. If in the future you get better and contact them again, it may be possible that you'll get them to agree to do trade with you after they've seen your development and would like your evolved style in their port.
Option 3: Move on. This is generally my preferred option. Once they've expressed no interest in working with me or are only wanting money with no real interest in shooting with me, I tend to want to put my energy toward those that want to work with me
Ultimately it's about getting to a win/win situation. It may be that the Model doesn't need to do trade work or have other means of getting fresh images to promote themselves. It may mean that you offset their costs for doing the shoot (travel, gas, hotel stay, etc.) as a means of barter. Perhaps you can get another Photographer and you all share the Model's half day rate. Try to think about how you can get to something that works for all involved. If you can't work something out, be mature and understand that you may not be able to come to terms AT THIS TIME. Leave the door open for future negotiations. If you show your ass they will remember and will share it with other Models. They have lists and they know who the assholes and douche bags are like this guy