The iPhone is the only iOS smartphone (older versions like the 3G, 3GS and 4 (the current version is 4S) are cheaper).
Android phones can be got very cheaply, but there can be hardware trade-offs (e.g. low quality screens), and many cheaper Android phones have outdated versions of Android, although this can be mitigated by installing a custom ROM like CyanogenMOD (a word of warning: depending on the specific phone, the difficulty of installation of custom ROMs ranges from very easy & safe to near impossible & risky. Something to keep in mind.)
The iPhone is the only iOS smartphone (older versions like the 3G, 3GS and 4 (the current version is 4S) are cheaper).
Android phones can be got very cheaply, but there can be hardware trade-offs (e.g. low quality screens), and many cheaper Android phones have outdated versions of Android, although this can be mitigated by installing a custom ROM like CyanogenMOD (a word of warning: depending on the specific phone, the difficulty of installation of custom ROMs ranges from very easy & safe to near impossible & risky. Something to keep in mind.)