No, it's not O.K. at all for you to do this.
You either invite someone or you don't. None of this, "You can bring someone but it'll cost you" nonsense.
If someone has a steady significant other, then they are a social unit and must be invited to the wedding as a pair. If you can't afford to feed both of them on your own dime, then do not invite either of them.
If someone has a steady significant other - a spouse, a fiance(e), a significant other that they live with, or a long-time partner (regardless of whether you've met them or not), then you write "Mr. Michael Smith and Miss Kelly Girlfriend" or "Ms. Nancy Andrews and Mr. Steven Fiance" or "Mr. and Mrs. John Jones" on the invitation envelope. If you don't know the partner's name, then call and ask. It is unacceptable to call a steady significant other "And Guest/Plus One." Call and get their names.
If the person is single but you'd like to let them bring a random date if they choose, then you can write "Ms. Ellen McDonald and Guest" on the invitation. Hopefully Ellen will respond with her guest's name (if not, call her and ask) and then you can write both names on her place card. If Ellen does not want to bring a guest then she should reply that only she is coming.
If you cannot afford for singles to bring a guest, then do not invite them with a guest. Do NOT tell them that they can bring a guest if they pay for them. You either host them or you don't.
If you choose to invite Ellen by herself, and Ellen replies that she is bringing a guest, then you need to call her and say, "I'm sorry for the confusion, Ellen, but we cannot accommodate your guest. Please let me know if you will still be coming to the wedding." Simple as that. If Ellen doesn't want to come without a date, then Ellen can stay home ... DO NOT tell Ellen that she can bring a date if she pays for him/her.