Be aware that most "bargain options" have tubes in them but it is not in a circuit that provides the "tube sound".
There are "starved plate" designs that run at lower voltages and there are many ways a tube circuit can be configured.
I have an HHB Radius 40 that I bought years ago, there are 3 tubes in it. It is useful but I don't consider it to have a tube sound. I don't believe it has an audio transformer in the circuit.
I also have a CAD M9 tube mic and while the voltage is there, they've got the tube in a circuit that does not deliver what I consider to be a tube sound either.
I also have a Focusrite ISA One that does not have tubes but has a very nice Lundahl transformer and it sounds more like a tube sound to my ears than either of the above options.
FWIW, I am a guitarist and have owned/own MANY tube guitar amplifiers - at least 30 different ones old and new. It is a bit of a different circuit that a tube mic pre but there is a certain sound that they both share. It isn't just having a tube, I think you need a tube and a transformer (or two), and you need to be able to "push" those components a bit to get the saturation and harmonics that are the tube sound.
Be aware that tubes are like lightbulbs, they have a life span and they will change and they will die. They will also sound different, you may or may not like the sound of a particular tube or you may love it until it changes. It may choose to die at an inconvenient moment.
A friend of mine who is also a guitarist and has owned/owns tube guitar amplifiers and he really likes the Soyuz Launcher for adding some character to his microphones. Another option might be putting a Tech 21 Sansamp into the insert path of a mic pre.
Something I have done and like is to create a duplicate track (copy and paste in my DAW) of a vocal track and then drop a guitar amp plugin into the second track and blend them. The IK Multimedia Fender 2 Collection for Amplitube sounds really good for that. I hope others post here as well, this is a great topic! Cheers, Kuru