You are a wonderful son thinking about your mom and upgrading her current collection. Also, welcome to our side of the forum -- we are nice to new people.
First, let me ask you -- what does she favor most out her cheapies? In order to give you a specifically targeted recommendation, we have to know where she comes from fragrance-wise, what notes or combinations she prefers.
Second, the idea that has never failed yet. Take her out to the mall where many lines are represented. Let her sniff as many scents as she can, encourage her to smell and speak about what she smells and feels, let her smell fragrances on you, smell fragrances on her. Flirt with sales assistants, make them work for you -- let them guess your relation to your mom (younger men in every woman's life can be puzzling, non? ), show affection, collect samples with glee to further sniff them at home. It is HER day that matters, not yours, OK? It is HER who would select her next big fragrance. Take her out to eat at a fancy sit down place. Mothers cherish experiences like that, she would remember your bonding day forever and eternity. She would say it is the best birthday gift ever.
Third, with Christmas coming -- gift her with a FB of scent she liked most on your sniffing/fun/bonding expedition. She will be wowed and swept off her feet, I am absolutely undoubtedly positive about that -- you remembered! All her sleepless nights were not in vain -- what a great MAN she grew.