It is unlikely that someone will just handle the suppression motion as once their appearance is entered they are in the case. If you don't pay them the balance of their fee then they have to file a petition to get out of the case. I'm sure the finances are hard but you would be well served to save and get funds where you can to hire counsel.
As for cost, I'd venture a pretty good guess that the information on fees you would get here would be worthless becuase 1. We know nothing about the case and 2. Fee can vary greatly depending upon experience and geography.
This information does not create an attorney /client relationship and should not be use or relied upon to make any decision in your case. Only consultation with your own attorney can provide you with the advice you need for your case.
It is uncommon but permissible. However, I would really attempt to avoid this situation. By the time of the suppression hearing, the attorney will have intimate details of your case and will begin your defense. If the suppression motion is denied, and you hire another attorney, that other attorney will have to review all the same documents the first attorney did, which would cost more money for you (assuming this is not a flat fee case).
I agree with my colleagues who advocate that you use the same attorney through the entire case. I write separately to add that it is not wise to select an attorney based solely upon price. Your freedom and reputation are at stake here. The impact of your decision can follow you the rest of your life. Now is not the time to save a few pennies by approaching your case piecemeal.
As to your second question, I can't speak for all attorneys, but fees in criminal cases are based upon the time and effort I anticipate will be required to prepare the client's defense. Unusual or complicated facts, or difficult clients, take more time and effort. Therefore, I must charge a higher fee. With almost three decades of criminal trial experience, my fees are probably higher than most, but less than some. Each set of facts and client is unique.
It is possible, but I would not recommend it, so long as you feel your attorney is fighting for you. Most attorneys fees will vary depending on the complexity of the case. Our office charges flat fees, which will vary depending on what stage of the case you are at and how complex the issues are. If you would like to discuss this in more detail, I can be reached at my office at 717-236-9391.