If not renewed, your domain will be hold for around 2 weeks, and you can get that domain with normal renewal fee+additional fee for holding (amount depends on different registrars). If you don't take it within this holding period, then domain will be deleted and will be available to buy again.
Yeah, check with your Registrar they may hold it for several weeks or just a few days. They might also keep the domain and charge you a whole load to get it back. Your registrar should tell you what will happen. You may also want to contact them if you can't pay immediately, before doing it. They may sympathise with you.
It usually takes a day or two for re-activation if caught in time. Once you have access to your dns records, check them plus MX records. They may have been reset.
How much in average is that fee? i just bought yesterday a domain and found out now that it existed 2 months ago, hope it has some traffic