It is probable after a time has passed with a missing tooth that there will not be enough room for a standard implant. Mini-implants are a possibility. Cost can be from $900 upward depending on the part of the country you live in. Remember there will also be the cost of the restorative portion that goes on top of the implant, ranging from $1000 to $2500 (again depending on the part of the country you are from.) The second solution is some sort of bridge. There is the Encore bridge with bonded retainers on the inner side of the teeth, with a veneer placed on the smile side. Costs here can be $1500-$4000. Finally you can have a removable partial denture, something that hooks on the palatal side of your teeth holding a tooth where the missing tooth is. This cost should be under $1000. Missign teeth can be a mystifying area of dentistry ~ hopefully this explanation has shed some light on it.
Gap too small for an implant
Your first option to consider may be orthodontics to create more space and move the adjacent teeth roots out if the way to make an implant restoration a possibility. As usual, your post gives totally inadquate information eg. which tooth, condition of adjacent teeth, bite pattern, lip line, other crowding, diemension of space available etc etc etc and you need diagnositc information from a dental examination to evaluate your options comprehensively. In some cases we can overcontour the adjacent teeth with bonding composite and this disguises the size of your small gap and improves the appearance for your smile (at negligable cost). a 3mm diameter implant is the smallest implant size that would provide a reliable dental implant for a crown, but of course you need a larger gap than this to fit it in! Don't worry, this is not a very challenging case, find yourself a good local dentist who has an interest in dental implants, works with orthodontists and is recommended by your colleagues and best of luck.