The cheapest computers with Alder Lake N CPUs, like ODROID H4 and many others, have only a single SODIMM socket and they are limited to 48 GB (which works, despite Ark advertising a limit of only 16 GB).
However there are many NUC-like small computers made by various Chinese companies, with AMD Ryzen CPUs and with 2 SODIMM sockets, in which you can use 64 GB of DRAM.
Those using older Ryzen models, like the 5000 series, may be found at prices between $220 and $350. Those using newer Ryzen models, up to Zen 4 based 7000 or 8000 series, are more expensive, i.e. between $400 and $600.
While there are dozens of very cheap computer models made by Chinese firms, the similar models made by ASUS or the like are significantly more expensive. After the Intel NUC line has been bought by ASUS, they have raised its prices a lot.
Even so, if a non-Chinese computer is desired and 64 GB is the only requirement, then Intel NUCs from older generations like NUC 13 or NUC 12, with Core i3 CPUs, can still be found at prices between $350 and $400 (the traditional prices of barebone Intel NUCs were $350 for Core i3, $500 for Core i5 and $650 for Core i7, but they have been raised a lot for the latest models).
EDIT:
Looking now at Newegg, I see various variants of ASUS NUC 14 Pro with the Intel Core 3 100U CPU, which are under $400 (barebone).
It should be noted that this CPU is a Raptor Lake Refresh and not a Meteor Lake CPU, like in the more expensive models of NUC 14 Pro.
This is a small computer designed to work reliably for many years on a 24/7 schedule, so if reliability would be important, especially when using it as a server, I would choose this. It supports up to 96 GB of DRAM (with two 48 GB SODIMMs). If performance per dollar would be more important, then there are cheaper and faster computers with Ryzen CPUs, made by Chinese companies like Minisforum or Beelink.
However there are many NUC-like small computers made by various Chinese companies, with AMD Ryzen CPUs and with 2 SODIMM sockets, in which you can use 64 GB of DRAM.
Those using older Ryzen models, like the 5000 series, may be found at prices between $220 and $350. Those using newer Ryzen models, up to Zen 4 based 7000 or 8000 series, are more expensive, i.e. between $400 and $600.
While there are dozens of very cheap computer models made by Chinese firms, the similar models made by ASUS or the like are significantly more expensive. After the Intel NUC line has been bought by ASUS, they have raised its prices a lot.
Even so, if a non-Chinese computer is desired and 64 GB is the only requirement, then Intel NUCs from older generations like NUC 13 or NUC 12, with Core i3 CPUs, can still be found at prices between $350 and $400 (the traditional prices of barebone Intel NUCs were $350 for Core i3, $500 for Core i5 and $650 for Core i7, but they have been raised a lot for the latest models).
EDIT: Looking now at Newegg, I see various variants of ASUS NUC 14 Pro with the Intel Core 3 100U CPU, which are under $400 (barebone).
It should be noted that this CPU is a Raptor Lake Refresh and not a Meteor Lake CPU, like in the more expensive models of NUC 14 Pro.
This is a small computer designed to work reliably for many years on a 24/7 schedule, so if reliability would be important, especially when using it as a server, I would choose this. It supports up to 96 GB of DRAM (with two 48 GB SODIMMs). If performance per dollar would be more important, then there are cheaper and faster computers with Ryzen CPUs, made by Chinese companies like Minisforum or Beelink.