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I have a vintage Peugeot carbolite 103 frame which is really bent. The rear triangle is bent in the direction of the drive side to such an extent that the drive side crank arm is blocked by the frame and cannot spin a full circle.
When I look at it from the back it is very noticeable.

Is there any way it can be cheaply fixed and will be safe to ride?

Thanks

It can be fixed. I can't guess how much your local bike shop will charge but any good shop should give you an estimate plus or minus 10% of what they will charge so you will know what to expect.

Here is an article at Park Tool explaining how to align a frame. It is possible to align a frame at home and get it kind of close even without shop tools if you have some skill. Some have used a string to measure alignment instead of the shop tool.

Once it is fixed it should be safe to ride assuming the rest of the bike is safe (see my note below).

From Sheldon Brown

A steel frame can be straightened (within limits) if bent, and still remain safe and strong.

NOTE: I am saying that a steel frame that has been straightened is safe to ride assuming the rest of the bike is safe.

Since I can't assess the over-all condition of your bike I'm not saying that your bike is safe. Your local bike shop can make this assessment and would be able to speak to the over-all safety of your bike.

Steel frames are definitely straightenable, and it can be done safely (i.e. without weakening the material significantly) depending on how much they are bent of course.

Your rear triangle appears to be at the more extreme end of mal-alignment and may not be fixable, but the bike is not ridable now, so you don't have anything to lose.

You can survey local bike repair shops and get some quotes. Without knowing the full extent of the damage, what bike repair shops charge in your area we can't tell you if the fame can be fixed for a cost that you would you consider 'cheap'.

The good news is that frame straightening is something a home bike mechanic can tackle.
Re-alining the rear stays is a fairly common thing to do on old steel frames so there are many YouTube videos that show how to do it. RJ the Bike Guy seems to have a good one (linked below).

Before attempting to straighten the triangle yourself, it would be prudent to check for other mis-alignments or damage. Knowing how the frame was bent in the first place would be helpful, if it was run over by a car for instance you might reconsider the frame's integrity.

Make sure the main triangle is aligned, the head tube and seat tube should both lie on the plane bisecting the left and right sides of the bike, and not be 'twisted' relative to one another. You can check that with a bubble level (or an smartphone with a level app).

If either the seat or chainstays have a sharp bend in them the frame may not be repairable (I can't see any sharp bends in the pictures so you are probably OK).