OK, that is a clone 60mm refractor on what is colloquially called the Mount of Doom. You will not need to use the diagonal at all, that will only place the DSLR sensor further out of the focuser. The nosepiece you have should fit directly into the empty focuser tube, however, what you have has an oversize flange on the end of the nosepiece which is likely causing it not to fit properly. The nosepiece I linked does not have a flange and will fit to a 1.25" focuser and lock into place properly once the thumbscrew is tightened. If your scope does not have a 1.25' internal diameter focuser you need the adapter linked below. Regardless, you still need the nosepiece without the end flange I linked earlier.
https://www.amazon.com.au/Solomark-0-965-Telescope-Eyepiece-Adapter/dp/B00OXKG8BS
Note that it may be difficult to achieve balance with your DSLR attached to the scope, given the additional weight. This will cause stability and targeting problems making the exercise frustrating and difficult.
TL;DR:
How cheap? :-)
For $30 you might get a cheap toy telescope to use for parts.
Else...
Quality
The cheapest good eyepieces are Plössl-type (52° apparent field of view) or the gold-line (various names, 66°, not to be confused with the poor 62°).
These cost $8-$15 (from China via eBay, Aliexpress) or $10-$25 (from the US).
Under focal-lengths of 10mm they have poor eye-relief.
You can get cheap sets of multiple eyepieces for $10-$20 but they are usually poor quality, sometimes even plastic.
0.965" focuser to 1.25" eyepiece adapter 0 1 2 3 *
Cheaper plastic adapters exist if you can find them. You can make one. Use a film can as they fit 1.25" eyepieces, you just have to cut off one end and fit a 0.965" tube
OR...
You will need a diagonal (f the telescope points up you can't look from underneath without snapping your back). These exist with 0.965" on the telescope side, and 1.25" for the eyepieces; Often they are much more expensive though, especially the good ones. Example.
Cheaper ones,
You can often find eyepiece sets with diagonal. Tempting, but poor quality. Often even worse than what was originally included.
Either's not great. They will show something (e.g. moon), but don't expect overly good views.
You need at least two eyepeices- One for an overview, high magnification (moon, planets). Sometimes one or two in-between.
The focal length depends on what telescope you have. Can you look for a label or engraving?
My guess is that you have a 50/600mm or 60/700mm telescope.
The maximum magnification shouldn't be over twice the aperture (50mm, 70mm), ideally less. Else the image wil lget very dull.
E.g. a 6mm "gold-line" if you want something high quality. $17 at Aliexpress, $25-$30 via Amazon.
Magnification equals telescope's focal length divided by eyepiece focal-length
Overview eyepiece: 20-32mm Plössl (for about any telescope)
As for planets: 6mm works out with many telescopes to be about the maximum. 6mm Plössl type are cheap, eye-relief is very short (4mm or so). I personally don't like it.
Again, one of the bad ones. 1
Examples
50/500
50/600
70/700
...And one or two in-between. 9, 15mm gold-line, or Plössl
Cheap Sets with barlow and Plössl-type are better but usually overpriced - the 4mm is basically unusable, eye-relief is horrible. I would not buy this set.
Barlows double the magnification, there are some for $10.
So you could get a cheap 25mm Plössl (see above) and 3x barlow that increases magnification (equivalent ~8mm eyepiece). A cheap barlow will reduce the contrast though, so it's also not really a good solution, nor cheaper than getting a decent eyepiece for $17-$25. (Very random link)
(*Note if some other person finds this post and buys one for their telescope: Doesn't work with reflector telescopes due to short back-focus)