Three ways to build an oven, all have advantages and disadvantages.
1) Build a clay cob oven. Dig the clay yourself (free), costs nothing to build actual oven. Disadvantages: takes a long time to cure, will crack, will need protection from elements. Won't last more than a few years.
2) Build a dome from firebricks. Looks amazing. Lasts as long as you want it to. Functionally superb. Can salvage bricks cheaply. Disadvantages: Needs some skill, will take a few days to build.
3) Buy a precast clay dome (I did) No need to build brick dome. Durable. Functional. Disadvantages: Costs quite a lot. Still have to insulate underneath and all around for heat retention (and weather protection.
All of these require you to build a base. I used hollow breezeblocks, drystacked, then filled with concrete and rebar, then rendered. Base top built with concrete lintel and paving slabs. Used calcium silicate board underneath base. Built a brick arch round oven door. Built a mesh dome filled with rockwool around dome, then rendered with limeplaster mixed with vermiculite. Took about 3 days. Cost about a grand including oven dome.