There are reasons the MX60(W) is widely available and inexpensive.
- It's reliable, but not very fast.
- The wireless MX60W is a single-band device.
- To install OpenWrt on an MX60(W), you need to open the case, connect a UART cable, and have a TFTP server available. Not everyone has the temperament and supplies for this.
For these reasons, anything that's better than an MX60 will probably be at least somewhat more costly.
Here's what I personally have had success with:
Check Point L-50(W). Kirkwood platform, 10 Ethernet ports (the stock firmware treats them as WAN + DMZ + 8 LAN; OpenWrt by default sets it up as WAN + 9 LAN), and also single-band wireless (on the L-50W model; the L-50 proper is wired-only).
Sophos XG and SG series. These are full-blown commercial-grade x64 devices, so there's some hardware muscle (Intel Atom at a minimum, all the way up to the Xeon). There are rack-mounted models (model numbers above 200) and desktop models. I am guessing you're more interested in the desktop end of the spectrum. The higher-end eight-port desktop models (125 and 135) are popular in the pfSense community, and prices can reflect that. Here are a few less expensive four-port models:
XG 85(w) can theoretically run pfSense, but it ships with a 4 GB eMMC, so the pfSense community doesn't really care for it. The successor, XG 86(w) ships with 16 GB eMMC, which is the low end of the pfSense territory (so at least some pfSense users will consider it).
SG105(w) and SG115(w) (revisions 1 and 2) cannot run pfSense at all (they have a 32-bit BIOS, so the pfSense installer just can't get past it and freezes). The 64-bit OpenWrt, meanwhile, has no problems installing or running. I've been told that Revision 3 has a newer BIOS that has workarounds for this issue, but I didn't have a chance to verify it myself.
The important takeaway from the OpenWrt standpoint is, any SG or XG series device will run OpenWrt for x64 without a hitch. There's also the RED series, but those are built, if memory serves, on Freescale chips. I've seen OpenWrt installation instructions for the RED 15(w), but I haven't tried them myself, nor do I know how applicable they are to other RED models.
Then, there's Cyberoam (which has been bought by Sophos a few years back). Lots of x64 hardware; the units I saw had AMD processors in them. Once again, the higher-end later models can be scooped up by the pfSense enthusiasts, but lower-grade units that run off CF cards or have BIOS that pfSense doesn't like are obtainable.
TrustWave has models that are similar to Cyberoam's (probably because they are made by the same company in Taiwan).
Mini-PCs can be interesting. There are dual-RJ45-port models (Intel NUC and clones, Zotac Zbox, etc.), but you have to assume that you will need to replace the Wi-Fi card (the cards installed by the manufactures are unlikely to support the access point mode).
A related category is thin clients. Some (but not all) models are expandable, so you can install a two-port or even four-port Ethernet card in addition to the already present single RJ-45. The warning on the Wi-Fi cards still applies though. Examples include HP t610 Plus, HP t620 Plus, and the Fujitsu Futro family. Since you mentioned a few quid, I am assuming you're in the UK, so you probably won't find a whole lot of HPs (they tend to be more prevalent in the North American market) but will see a lot of Fujitsu units well within reach, especially if you're willing to buy from German and Dutch sellers.